GMAT Math Practice Test #1

Answer B

Assume the number is n.

n x 25% = n x 30% - 15
or 0.25n = 0.30n -15
or 0.05n = 15
or n = 300

Key takeaway: First you convert the word problem into a simple equation with a variable. Then solve simple equation getting the variable alone on one side of the equal sign.
Remember some important percent and decimal equivalents :

1% = 0.01= 1/100 10% = 0.1 = 0.10 =1/10 25% =0.25 = ¼ 50% =0.50 = 1/2