Probability

Probability

Relative frequency, Venn diagrams, mutually exclusive and complementary events.

CAPS

Grade 10 · Mathematics · Probability

This sits on Mathematics Paper 1.

  • The paper

    Paper 1 is algebra, exponents, equations, number patterns, functions, finance and probability. Copy the formula, then substitute.

  • Cognitive level

    CAPS spreads the marks across knowledge, routine procedures, complex procedures and problem solving. A routine question still wants the line of working. A problem-solving question wants you to choose the method, not only to calculate.

  • The relation

    P(A) = n(A)n(S). Classical probability

  • How a marker reads it

    Leave probability as a simplified fraction unless a decimal is asked.

Each Mathematics paper is 150 marks. A correct substitution can earn a method mark even when the last arithmetic slips, if the formula was the right one.

Diagram

Worked example

Bag: 5 red, 7 blue. One drawn. P(red)?

Before any working — hunt first

Given

Numbers and units already in the sentence: 5 red · 7 blue. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: P(red). Circle that word — that is the missing variable.

Find

We are required to find: P(red). Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write P(A) = n(A)n(S) with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives P(red).

Simplified approach

Favourable over total. 512.

Culture note

A lucky-dip at a Tzaneen market stall uses the same counting idea.

Formulas

P(A)=n(A)n(S)

Classical probability

How to hunt the missing letter

Do not start by naming the letters. First hunt the missing one: copy every number you can see (Given). The letter with no number is Missing. The asking word is Find. The formula that still contains that letter is what you Calculate.

How to hunt: (1) Underline every number and its unit — those are Given. (2) Circle the asking word — find, calculate, how far, what is. That word names Find. (3) The letter the asking word names, which has no number yet, is Missing. (4) Write the formula with empty letters. Cross out every given letter. The letter still standing is what you Calculate.

  • A

    an event

    How you spot this in the sentence

    When the sentence names the an event, that letter is A. Copy the number next to it into Given. If the sentence never names it, A is Missing. The outcome you care about (e.g. drawing a red card).

    Example:A = “the coin lands heads”.

  • n(A)

    favourable outcomes

    How you spot this in the sentence

    When the sentence names the favourable outcomes, that letter is n(A). Copy the number next to it into Given. If the sentence never names it, n(A) is Missing. How many ways A can happen.

    Example:A fair die, A=even: n(A)=3 (2, 4, 6).

  • n(S)

    sample space size

    How you spot this in the sentence

    When the sentence names the sample space size, that letter is n(S). Copy the number next to it into Given. If the sentence never names it, n(S) is Missing. How many equally likely outcomes exist in total.

    Example:A die has n(S)=6.

  • P(A)

    probability

    How you spot this in the sentence

    When the sentence names the probability, that letter is P(A). Copy the number next to it into Given. If the sentence never names it, P(A) is Missing. n(A) / n(S). Always between 0 and 1.

    Example:Even on a die: P(A)=36=½.

Exam tricks you must know

These are the moves markers look for — not shortcuts that skip the method, but the habits that earn the method mark.

  • “Factorise fully” means HCF and then the identity — leaving 2(x² − 9) loses a mark

    How:Look at the given polynomial. Pull out the biggest number and letter that divides every term (HCF). Then look at what is left: difference of squares, trinomial, or grouping. Both layers go on the page. Stop only when nothing else factorises.

    When to use it:Any “factorise fully” on Paper 1, especially KZN.

  • Memorise exact values for 0°, 30°, 45°, 60°, 90° — compound angles collapse without them

    How:Before a calculator, write the triangle: 30-60-90 and 45-45-90. sin 30° = 12, sin 45° = 1√2, sin 60° = √32, and the cosines swap. Compound-angle questions want these as fractions with division lines, not decimals.

    When to use it:Reduction, compound angles, and “without a calculator” trig.

  • Name similar triangles in corresponding order: ΔABC ∼ ΔDEF, not ΔABC ∼ ΔEFD

    How:Hunt equal angles first and mark them on the sketch. Then list vertices so that equal angles sit in the same position: A maps to D, B to E, C to F. The ratio statement uses that same order. Wrong order loses the reason mark even if the lengths are right.

    When to use it:Similarity, intercept, and 3-D trig that shares a triangle.

Step 1 / 8

Step A — Read the question twice

Slow down. Circle the word that names the unknown. Underline every number and its unit. Do not grab a formula yet — first decide what the paper is actually asking.

Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

P(a fair die, rolling a 6)?

4. Step-by-step working

  1. 01

    Write the formula

    P(E)=n(E)n(S).

    P=n(E)n(S)
  2. 02

    Substitute

    P=16=0.17.

    16

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: P(a fair die, rolling a 6)?

Before any working — hunt first

Given

Numbers and units already in the sentence: 6. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: P(a fair die, rolling a 6). Circle that word — that is the missing variable.

Find

We are required to find: P(a fair die, rolling a 6). Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write P = n(E)n(S) with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives P(a fair die, rolling a 6).

In other words — as if you have never seen this before

If you have never seen this before: P = favourabletotal. A probability sits in [0; 1]. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

Favourable = 1, total equally likely = 6.

3. Which method — and why this one?

P = favourabletotal. A probability sits in [0; 1].

4. Step-by-step working

  1. 01 / 05

    Write the formula

    P(E)=n(E)n(S).

    P=n(E)n(S)

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  2. 02 / 05

    Substitute

    P=16=0.17.

    16

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  3. 03 / 05

    Step G — Sense-check the size

    Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.

    Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.

  4. 04 / 05

    Step F — Box the answer with its unit

    16

    Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.

  5. 05 / 05

    Step H — What the marker is looking for

    On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.

    Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

16

Copy onto your answer sheet — in this order

Formula: P=n(E)n(S)

Answer: 16

6. Check it like a marker

0 ≤ 16 ≤ 1. Impossible events are 0; certain events are 1.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

Ntsako · fx-991ZA PLUS. Tap to calculate.