If 64 feet of rope weigh 20 pounds how much will 80 feet of the same type of rope weigh

It will weigh 25 pounds.

I made this into a proportion where 20 pounds/64 feet = x pounds/80 feet. I simplified 20/64 to get 5/16. To get from 16 to 80, you'd have to multiply it by 5. To make it proportional, you would also have to multiply the numerator by 5 as well, to get x, which eventually turns out to be 25.