Difference between revisions of "Probability qn 1"

Revision as of 21:49, 16 December 2013

The probability that a number selected at random between 100 and 999 (both inclusive) will not contain the digit 7 is:

(a)16/25 (b)(9/10)^3 (c)27/75 (d)18/25

Solution

First digit can be chosen in 8 ways from 1-9 excluding 7
Second digit can be chosen in 9 ways from 0-9 excluding 7 and similarly the third digit in 9 ways.
So, total no. of ways excluding 7 = 8*9*9
Total no. of ways including 7 = 9 * 10 * 10
So, ans = (8*9*9)/(9*10*10) = 18/25