Question

A mathematics book has 360 pages, of which 50% are on ALgebra, 20% on Geometry and the remainder on Arithmetic. How many pages of arithmetic are there in the book?

I'm seeing that the answer is 108 pages but I can't understand how to arrive at that answer.
Steve
you have 70% of the pages accounted for. So, if there are 30% left, then
.30*360 = 108