To find the answer of 5/9 of 72 you just have to multiply both together.
To make it easier, think of 72 as 72/1, it is still the same thing but if you are working with two 'fractions' you may find it easer.
So it becomes 5/9*72/1
You times the numerator together (5*72) which is 360
You times the denominator together (9*1) which is 9
Therefore it is 360/9, however you can simplify this by 360 dividing 9
which is 40. Therefore, 5/9 of 72 is 40.