Hi Adam,

a - b is the difference between a and b or the change in going from b to a. I am thinking of a being larger than b so that this quantity has value b and then increases to value a, hence the increase is a - b. For example consider b = 12 and a = 18 then a - b = 6. Dividing by b gives a measure of this increase relative to the value of b. In this case (a - b)/b = 6/12 = 1/2, or said as a percentage, 50%. This is the quantity we call percentage increase or percentage change.

For example look at this question about car sales.

Penny