Comment: Though this problem is one of only two examples given relating to strand 5 of Project Maths. The solution to this problem requires the use of Differentiation Methods (either parametric or implicit) that are no longer on the LCHL Course!

However, since this question was in the 2014 Sample Paper it is important. Here is a similar question about an increasing sand pile from Roantree's notes. If you understand this you should be OK.