其它
Pip and Bud buys some £$$10$$ books and £$$15$$ books together. If they spent £$$90$$ on the books, how many books have they bought at most in total? 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Indefinite Equations
Let $$x$$ be the number of £$$10$$ books and $$y$$ be the number of £$$15$$ books  $$10x+15y=90$$, which has two positive integer solutions only.  $$\begin{cases} x=6 \textbackslash\textbackslash{} y=2 \end{cases}$$, $$\begin{cases} x=3 \textbackslash\textbackslash{} y=4 \end{cases}$$  $$6+2=8$$ 