help please ...here is a question that i got stuck in..

Can you please help me solve it..

((a manager of a supermarket usually adds a mark-up of 20% to the wholesale prices of all the goods he sells.he reckons that he has a loyal core of f customers and that<if he lowers his mark-up to x% he will attract an extra k(20-x) customers from his rivals.each week the average shopper buys goods whose wholesale value is a pounds.show that with a mark-up of x% the supermarket will have an anticipated weekly profit of

1/100ax((f+20k)-kx)) pounds

show that the manager can increase his profit by reducing his mark-up below 20% provided that 20k>f