If you pretend 20 is x, you solve the equation by pluging in x (20) into the equation and it gives you 70, 70 is what the big part is and 70+20 is 90, which is the size of a 90degree side.
It is 9 because 1/2 goes into 3, 6 times, and then 3/4 divided by 1/2 is 1 1/2 so 1/2 goes into 3/4 1/12 times, so then figure out how mnay 1/2 is in 1 1/2 and youll get 3 so 6 plus 3 equals 9 :)