Find the perimeter of an isosceles triangle with base length 20 and area 240.

1/2 of the area  = 120

1/2  of the base  = 10     ......so.........

A = (1/2)BH

120 = (1/2)(10)(H)

120/5  = H   =  24

And one of the equal sides  = sqrt [ H^2 + (1/2 of the base)^2 ] =  sqrt [24^2 + 10^2]  = sqrt(676)   = 26

So....the prerimeter=   2*26 + 20  = 72 units