Nova Book

[sana.noor]

what is the probability that the sum of two different numbers randomly picked (without replacement) from the set S = {1,2,3,4} is 5
a)1/5
b)3/16
c)1/4
d)1/3
e)1/2

d
explanation: 4.3 = 12 ways to select two numbers
where as 4 ways to select two numbers that sum to 5 (1+4, 4+1, 2+3, 3+2)
4/12 = 1/3
Is their any other way to do this question?

this question is from Nova Gmat math bible...is it a good book for practice? the book is poorly edited but can i use it as a supplement?

[Anurag@Gurome]

Is their any other way to do this question?

The method provided in the book is short and simple enough.
You can try for 1 - [Not getting a sum of 5], but that will lead to a bit more calculation.

Or you can try the following method which is almost same as the book.
Number of ways to select 2 numbers from 4 = 4C2 = 6
Number of ways to select 2 numbers from the set such that there sum is 5 --> {1 and 4} or {2 and 3} = 2

Hence, required probability = 2/6 = 1/3

The correct answer is B.