Setup some simple formula describing what they are doing.
For example, if you want to find the number of years it takes to save 200Rs given all the information in the problem, you could setup something like:
[tex]
32 + (32 + 4) + (32 + 4 + 4) + ... + (32 + 4N) = \sum_{n=0}^N (32 + 4n) = 200[/tex]
and solve for N.