18. Write down a formula for each situation: (a) A plumber has a fixed call-out charge of $80 and has an hourly rate of $60. Work out the total charge, C, for a job that takes L hours in which the cost of materials and parts is $K. (b) An airport currency exchange booth charges a fixed fee of $10 on all transactions and offers an exchange rate of 1 dollar to 0.8 euros. Work out the total charge, C, (in $) for buying x euros. (c) A firm provides 5 hours of in-house training for each of its semi-skilled workers and 10 hours of training for each of its skilled workers. Work out the total number of hours, H, if the firm employs a semi-skilled and b skilled workers. (d) A car hire company charges $C a day together with an additional $c per mile. Work out the total charge, $X, for hiring a car for d days and travelling m miles during that time.