UPSC NDA I 2016 Mathematics Solved Paper

As we know that 3 dice are thrown. We want prob. of sum on three faces at least 5 i.e. some may be 5 or more. We will find prob. of sum on three faces not 5 or less, i.e. sum on faces is 3 and 4 (1,2 is not possible because of 3 dice).
No. of ways for sum on faces not 5 or more =4
Total out comes=216
Prob. of not 5 or more =

4

216

Prob. of sum on three faces at least 5
1−

4

216

=

212

216

=

53

54