5. Paul is painting a wall. He knows the area of the wall is 1920 square metres, correct to the nearest ten. He uses tins of paint, each of which can cover 18 square metres, correct to the nearest integer.
He needs to paint the wall completely, and still have at least half a tin of paint left over for any minor repairs.
What is the smallest number of tins he must buy to be sure of having enough paint?
SolUtion
111

The number of tins required to certainly cover the wall will be the maximum possible size of the wall divided by the minimum possible amount of paint per tin, i.e. $\frac{1925}{17.5}=\frac{19250}{175}=\frac{770}{7}=110$ (exactly). Because the calculation uses an exact number of tins, and as he requires a surplus of half a tin for any touch-ups he should buy one additional tin. Therefore, he should buy 111 tins of paint.