The sum of the squares of three numbers is 138, while the sum of their product taken two at a time is 131. Their sum is :

Answer: Option A

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 × 131
= 400
⇒ (a + b + c) = $$\sqrt{400}$$  = 20