Let surface area of the spherical balloon A=4πr2
dtdA=8πrdtdr=k (let) ...(1)
On integrating on both sides w.r.t t , we get
4πr2=kt+C .
Given that, at t=0,r=3.
⇒36π=C
Also given that, at t=5,r=7
⇒4π×49=5k+36π
⇒5k=4π(49−9) ⇒5k=4π×40
⇒k=32π
On substituting k value in equation (1)we get, 4πr2=32πt+36π
⇒r2=8t+9
Given t=9.
⇒r2=81⇒r=9.