das is disguised pythagorean theorme
the height is one leg
the distance from house is other leg
length of ladder=hyptoonuse
so
remmeber
if legs of right triangle are a and b and hyptonuse is c then
a²+b²=c²
so
legs are 15 and 20
15²+20²=c²
225+400=c²
625=c²
sqrt both sides
25=c
25ft is length of laddder
