Given:
Rate of increase in ripple radius,
= 2 ft/sec
Solution:
Area of circle =

Differentiating w.r.t 'r', we get:
= 2
Rate of change in area is given by:
=
.

(by chain rule)
= 2
.

when radius, r = 5ft
= 2
. 2 = 20

Now,
when r = 10 ft
= 2
.

= 2
. 2 = 40