1.  Of the three numbers, the sum of the 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 :
A. 3
B. 20
C. 30
D. 25

Answer: Option C

Explanation:

Let the number be x, y and z.

Then, x + y = 45, y + z = 55 and 3x + z =90

∴ y = 45 - x,
⇒ z = 55 - y = 55 - (45 - x) = 10 + x.
⇒ 3x + 10 + x = 90 or x = 20.
⇒ y = (45 - 20) = 25 and
∴ z = (10 + 20) = 30.
Third number = 30.