A bag has 6 red, 9 yellow and 5 blue balls. One ball is drawn at random. Find the probability that it is (i) yellow, (ii) not red. (iii) If 5 more blue balls are added, find the probability of drawing a blue ball.

1. 1 · Write what is given and what is to be found. 6 red, 9 yellow and 5 blue balls. To find: P(yellow), P(not red), and P(blue) after 5 blue balls are added.
2. 2 · Find the total. Total = 6 + 9 + 5 = 20 balls.
3. 3 · Write the formula. P(E) = favourable outcomes ÷ total outcomes.
4. 4 · Work out (i) and (ii).
   • P(yellow) = 9/20.
   • P(not red) = (9 + 5)/20 = 14/20 = 7/10.
5. 5 · Work out (iii). New total = 20 + 5 = 25; blue balls = 5 + 5 = 10. P(blue) = 10/25 = 2/5.
6. 6 · Check and box the answer. Check (ii): P(red) = 6/20 = 3/10, and 1 − 3/10 = 7/10. Box all three answers.

Scoring tip: When balls are added, write the new total on its own line. Forgetting to change the total is the usual slip.