2015 Gauss 8 Problem 15

Two puppies, Walter and Stanley, are growing at different but constant rates. Walter's mass is 12 kg and he is growing at a rate of 2 kg/month. Stanley's mass is 6 kg and he is growing at a rate of 2.5 kg/month. What will Stanley's mass be when it is equal to Walter's?

\textbf{(A)}\ 24\,\text{kg}\quad \textbf{(B)}\ 28\,\text{kg}\quad \textbf{(C)}\ 32\,\text{kg}\quad \textbf{(D)}\ 36\,\text{kg}\quad \textbf{(E)}\ 42\,\text{kg}

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Gauss contests here.