If the sum of two numbers is 33 and their difference is 15, the smaller number is :__________?

A. 9
B. 12
C. 15
D. 18
Let the numbers be x and y.
Then, x + y = 33     …(i)
and x – y = 15     …(ii)
Solving (i) and (ii), we get : x = 24, y = 9.
∴ Smaller number = 9.