A6. Tea and a cake cost $£ 4.50$. Tea and an éclair cost $£ 4$. A cake and an éclair cost $£ 6.50$. What is the cost of tea, a cake and an éclair?
SOLUTION $\square$
Let the costs, in pounds, of tea, a cake and an éclair be $t, c$ and $e$ respectively. Then $t+c=4.5, t+e=4$ and $c+e=6.5$. Adding these three equations gives $2 t+2 c+2 e=15$. So the cost of tea, a cake and an éclair is $£ 15 \div 2=£ 7.50$.