A, B and C are three numbers such that $A+B=10, B+C=19$ and $C+A=21,$ then number A, B and C respectively are

6, 4, 15

The correct answer is Option 3: 6, 4, 15

Add all three equations:
(A + B) + (B + C) + (C + A) = 10 + 19 + 21
This gives: 2(A + B + C) = 50 ⇒ A + B + C = 25

Now find each value:
A = 25 − (B + C) = 25 − 19 = 6
B = 25 − (C + A) = 25 − 21 = 4
C = 25 − (A + B) = 25 − 10 = 15