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Problems on Numbers - General Aptitude

Q1:

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

ANS:A - 20

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.