Weekly Problem 41 - 2009
Both $£4.20$ and $£7.70$ are multiples of
$70p$, so $£C$ must also be a multiple of $70p$. The
only multiple of $70p$ between $£90$ and
$£96$ which is a whole number of pounds is
$£91$.
$7$ children and $8$ adults cost $7\times £4.20+8\times
£7.70=£29.40+£61.60=£91$.
Or
$18$ children and $2$ adults cost $18\times £4.20 +
2\times £7.70 = £75.60 +
£15.40 = £91$.