1.  The sum of the squares of three numbers is 138, while the sum of their products taken two at a time is 131. Their sum is :
A. 20
B. 30
C. 40
D. None of these

Answer: Option A

Explanation:

Let the numbers be a, b and c. Then,

a2 + b2 + c2 = 138 and (ab + bc + ca) = 131.

(a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca)

= 138 + 2 x 131 = 400

(a + b + c) = 400 = 20