Question

how many feet of fencing will you need to fence in a rectangular yard 20 feet by 30 feet, assuming that you use the wall of a 30-foot-long building for one side?

Answer 1

like 70 to 100 feet of fencing i think

Answer 2

This yard is a rectangle of dimensions 20 ft x 30 ft.  Meaning you have 2 sides of 20 feet and 2 sides of 30 feet.  This would be 100 feet, but you are using 30 feet of a building instead of fence on one side.  Therefore it is 100 feet - 30 feet.  The final answer is 70 feet total.