Aspherical snowball is melting in such a way that its diameter is decreasing at rate of 0.2 cm/min. at what rate is the volume of the snowball decreasing when the diameter is 13 cm

Dd/dt = -0.2 cm/min 
d = 10 cm 

Next, you need to know the volume formula for a sphere. 

V = 4πr³ / 3 

The last piece of information you need to know is that the diameter is twice the radius (should already know this). Rewrite the equation in terms of diameter. 

V = 4π(d/2)³ / 3 

Now we can take the derivative of the equation with respect to time t. (Note that "dd/dt" means "the derivative of the diameter with respect to time".) 
dV/dt = (1/3)(4π)(1/8)(3d²)(dd/dt) 

Substitute in everything that you know. 
dV/dt = (1/3)(4π)(1/8)(3(10)²)(-0.2) 

Now just multiply everything to get the decrease in volume with respect to time. 
dV/dt = -31.416 cm/min <= FINAL ANSWER 

You're answer was only wrong because you forgot the negative sign (its diameter is decreasing and so is the volume). I hope this helped.