Past papers › JAMB 1998 · UME › Question 46 Question JAMB General Maths 1998 Objective Probability Probability
A bag contains 16 red balls and 20 blue balls only. How many white balls must be added to the bag so that the probability of randomly picking a red ball is 2 5 \frac25 5 2
Worked solution (try it first) Let
w w w white balls be added.
The total becomes
16 + 20 + w = 36 + w 16 + 20 + w = 36 + w 16 + 20 + w = 36 + w .
So
16 36 + w = 2 5 \frac{16}{36 + w} = \frac25 36 + w 16 = 5 2 .
Cross-multiply:
80 = 72 + 2 w 80 = 72 + 2w 80 = 72 + 2 w .
Take 72 from both sides:
2 w = 8 2w = 8 2 w = 8 , so
w = 4 w = 4 w = 4 , option A.
Watch out
The total includes the 20 blue balls. Writing 16 16 + w = 2 5 \frac{16}{16 + w} = \frac25 16 + w 16 = 5 2 w = 24 w = 24 w = 24 Report a problem with this question