GMAT Math Practice Test #1

Answer: C -First find LCM which is exactly divisible by 12, 15, and 20

12 = 2x2x3x3
15= 3x5
20= 2x2x5
LCM = 2x2x3x3x5.

In the LCM 2 and 3 have their pairs, but 5 are alone. Pairs are perfect squares. So we have to multiply LCM by 5 to make it perfect square.

2x2x3x3x5x5 =900 is the smallest perfect square number

Key takeaway: The Least Common Multiple (LCM) is the smallest number that is a multiple of two or more numbers. Write each number as products of their prime factors.