Related rates, Differential equations calculus.

Please help.

Once a spherical shaped soufflé is put in the oven, its volume increases at a rate proportional to its radius.

1. Show that the radius r cm of the soufflé, at t minutes after it has been put in the oven, satifies the differential equation dr/dt = k/r , where k is a constant.

2. Given that the radius of the soufflé is 8 cm when it goes in the oven, and 12 cm when it´s cooked 3 minutes later, find, to the nearest cm, its radius after 15 minutes in the oven.

Thanks!