Apair of fair dice is cast. what is the probability that the sum of the numbers falling uppermost is less than 4, if it is known that one of the numbers is a 2

The ways to make a 7 with 2 die is:
(1) 6 and 1
(2) 1 and 6
(3) 2 and 5
(4) 5 and 2
(5) 3 and 4
(6) 4 and 3
There are 6 ways to make a 7 with the 2 die,
but there is only 1 way to make 7 if the 1st
die comes up 6.
The probability of getting
(1) 6 and 1 is 1/6

The answer for you question is b