13.4.1 Simple Probability, PT3 Focus Practice


Question 1:
A box contains 5 pink marbles and 21 yellow marbles. Sharon puts another 4 pink marbles and 1 yellow marble inside the box. A marble is chosen at random from the box.
What is the probability that a pink marble is chosen?

Solution:
Total number of pink marbles = 5 + 4 = 9
Total number of yellow marbles = 21 + 1 = 22
Total number of marbles = 9 + 22 = 31

∴ P ( pink marble ) = 9 31


Question 2:
In a group of 80 prefects, 25 are girls. Then 10 boys leave the group.
If a prefect is chosen at random from the group, state the probability that the prefect chosen is a boy.

Solution:
Number of boys = 80 – 25 = 55

After 10 boys left,
Number of boys = 55 – 10 = 45

Total number of prefects = 25 + 45 = 70
∴ P ( b o y ) = 45 70 = 9 14


Question 3:
60 people have taken part in a singing contest. If a person is chosen at random from all the contestants, the probability of getting a male contestant is 8 15 . If there are 12 males and 3 females who did not qualify to the second round, find the probability that a male is chosen from the second round contestants.

Solution:
Number of male contestants = 60 × 8 15 = 32

After 12 males and 3 females left,
Total number of contestants left = 60 – 15 = 45

Number of male contestant = 32 – 12 = 20
∴ P ( male contestant ) = 20 45 = 4 9


Question 4:
John has a collection of stamps from Thailand, Indonesia and Singapore. He picks one stamp at random. The probability of picking a Thailand stamp is 1 5  and the probability of picking an Indonesia stamp is 8 15 .  John has 20 Singapore stamps. Calculate the total number of stamps in his collection. 

Solution:
Probability of picking a Singapore stamp = 1 − 1 5 − 8 15 = 4 15 Given that John has 20 Singapore stamps, ∴ Total number of stamp in John's collection = 15 4 × 20 = 75

Leave a Comment