Question 1182240
Given:
21 of 42 boxes have ring.
Once opened, cannot be replaced.
Find probability that two randomly selected boxes both have the ring.
 
Solution:
probability that the first one is a ring
= 21 out of 42 
= 21/42
= 1/2
probability that the second box is a ring 
= 20 out of 41
= 20/41
 
Probability that both of them are rings, we use the multiplication rule to get
P(R,R) 
= (1/2) * 20/41
= 10/41
= 0.24390... (approximately)
= 0.2439 (to four decimal places)