Eliot buys some~\textquotesingle Sour Sweets\textquotesingle{} bars which cost $12$p each.  He pays for them with three £$1$ coins.  Maya buys the same number of \textquotesingle Chocolate Drops\textquotesingle{} bars which cost $17$p each.  She pays for them with four £$1$ coins.  They each receive the same amount of change. Work out how many bars each person bought. 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation