mensuration,

A circular swimming pool is surrounded by a concrete walk 4m wide. If the area of the walk is 11/25 of that of the pool, then the radius of the pool is ..................................




Answer:

Π [(r + 4)² – r² ] = (11/25) Π r²

or, Π [ 8 ( r + 2) ] = (11/25) Π r²

or, 8 ( r + 2) = (11/25) r²

or, 200r + 400 = 11 r²

or, 11 r² – 200r + 20r – 400 = 0

or, (r – 20)(11r + 20) = 0

So,  r = 20, ignoring the negative value