I drew a diagram. The green region is where the goat can graze and the beige region is the outbuilding.
The partial circumference where the goat can graze is three quarters of the circumference of a circle of radius
40 ft. Do you know the circumference of a circle of radius 40 ft?
The part of the perimeter of the outbuilding that is inside the goats grazing area is two sections of length 40 ft each. How much of the perimeter of the outbuilding is left?