Of the three numbers, the sum of first two is 45, the sum of the second and the third is 55 and the sum of the third and thrice the first is 90. The third number is?

20
25
30
35

Correct Answer: 30

Explanation:

Let the number be x, y, z
Then,
x + y = 45,
y + z = 55,
z + 3x = 30
Now, y = (45 - x) and
z = 55 - y
⇒ z = 55 - (45 - x ) = 10 + x
∴ 10 + x + 3x = 90
⇒ x = 20
So, third number = z
= 10 + x = 30

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