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
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
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)
$=\frac{3}{6}=\frac{1}{2}$
P (a prime number)