Let diameter be D. Then .

Given: .

Common knowledge: . Therefore .

Therefore: .

You were given t = 12 minutes because that's the question!!! Find the value of when t = 12 minutes!

So you have to substitute the value of D at t = 12. Clearly and you know that . Therefore D = ....... at t = 12 minutes.

Therefore when t = 12 minutes is equal to ......