18. Dick Turnip sold his horse, Slack Bess, for $£ 56$. The percentage profit he made was numerically the same as the cost, in pounds, of his horse. What was the cost of his horse?
A $£ 36$
B $£ 40$
C $£ 45$
D $£ 48$
E $£ 50$
18. B Let the cost of Slack Bess be $£ C$. Then the profit on the horse is $C \%$.
Therefore $C\left(1+\frac{C}{100}\right)=56$. So $C^{2}+100 C-5600=0$. Hence $(C+140)(C-40)=0$. Therefore, as $C>0, C=40$. So Slack Bess cost $£ 40$.