If 80% is 72 marks, what is full marks?
\(Use\ a\ proportion:\\\begin{array}{ccc}80\%&-&72\\100\%&-&x\end{array}\ \ \ |cross\ multiply\\\\\\80\cdot x=72\cdot100\\80x=7,200\ \ \ |diviede\ both\ sides\ by\ 80\\\boxed{x=90}\leftarrow your\ answer\)