Find the base for the following items. 50 is 20% of what number? 50 is 20% of what number? 18 is 331⁄3% of what number? 4 is 1% of what number? 5% of what number is 90? 162⁄3% of what number is 72?

1) 50 is 20% of what number? 
50 = 0.2*x     Divide both sides by 0.2, or multiply each side by 5
x = 250

2) 18 is 33 and 1/3% of what number? 33 and 1/3% is equal to 1/3
18 = (1/3)x       Multiply each side by 3
x = 54

3) 4 is 1% of what number? 1% is the same as 1/100
4 = (1/100)x      Multiply each side by 100
x = 400

4) 5% of what number is 90? 5% is equal to 1/20
90 = (1/20)x
1800 =x

5) 16 2/3% of what number is 72?   16 and 2/3, or 16.666%, is equal to 1/6
72 = (1/6)x       Multiply each side by 6
x = 432

The answer would be 13.5 minutes.