The answer is 5 cakes.
EXPLANATION
To solve this, you work out that 1/4 of an hour is 15 mins 2/5 of an hour is 24 mins and that 3 1/4 hours is 195 mins.
Using these, we can work out that it takes 39 mins per cake (15 for frosting + 24 for decorating). When we divide 195 by this we get 5, meaning that the answer is 5 cakes.
Hope this helps!