A box contains a certain number of balls, marked successively from 1 to n. If there are 45 different ways that two balls can be selected from the box such that the ball with number 3 marked on it is not selected, then what is the value of n?
Answers.
A) 11
B) 10
C) 9
D) 8
E) 7
(n-1)C2=45   n-1=10   n=11   The answer is 11.
<result>A</result>
A box contains a certain number of balls, marked successively from 1 to n. If there are 45 different ways that two balls can be selected from the box such that the ball with number 3 marked on it is not selected, then what is the value of n?