You form an equation where x is the length of one of the equal sides in the isosceles triangle, which is where there's 2x, and the (x-1) is the third side which is shorter by 1 inch.
2x+(x-1)=20
3x=20+1
3x=21
3x/3=21/3
X=7
So the two equal sides are seven inches long, and because the third side is one inch shorter, it is 6.
7+7+6=20
Hope this made sense :)
