# 13.4.2 Simple Probability, PT3 Focus Practice

Question 5:
A box contains 36 green erasers and a number of red erasers.
If an eraser is picked randomly from the box, the probability of picking a red eraser is $\frac{5}{8}$ . Find the number of red erasers.

Solution:

Probability of picking a green eraser = 1 – $\frac{5}{8}$ = $\frac{3}{8}$

Total number of erasers in the box = 36 × $\frac{8}{3}$ = 96

Hence, number of red erasers in the box = 96 – 36 = 60

Question 6:
A box contains blue marbles and green marbles. A marble is picked at random from the box.
The probability of picking a blue marble is $\frac{6}{11}$ . If there are 30 green marbles in the box, how many blue marbles are there in the box?

Solution:

Probability of picking a green marble = 1 – $\frac{6}{11}$ = $\frac{5}{11}$

Total number of marbles in the box = 30 × $\frac{11}{5}$ = 66

Hence, number of blue marbles in the box = 66 – 30 = 36

Question 7:
Table below shows the distribution of a group of 90 pupils attending a penalty kick training programme.

 Form Four Form Five Girls 33 15 Boys 18 24
A pupil is chosen at random from the group to start the penalty kick training.
What is the probability that a boy from Form Five will be chosen?

Solution:
P (a boy from Form Five)
$=\frac{24}{90}=\frac{4}{15}$

Question 8:
Diagram below shows some number cards.

A card is picked at random. State the probability that a prime number is picked.

Solution:
prime number = {11, 19, 37}
P (a prime number)
$=\frac{3}{6}=\frac{1}{2}$