All you have to do to find what number a certain number is a certain percentage of is to divide the known number by the percentage that it is reduced by.
To do this, we must first convert the percentage into a decimal. To do this, we can either divide the percentage by 100, or (my personal shortcut) just shift the decimal point over two places to the left.
40% = 0.4
Now, we divide the known by the percentage/decimal.
72 / 0.4 = 180
72 is 40% of 180.
Hope that helped! =)