a machine that cost £35,000 is operated 8 hours a day in a 5-day week but one hour each day is used for test purposes. for use of the machine there are three scales of charges, the first at the rate of £5 an hour for private use, the second at £15 an hour for research work and the third at £40 an hour for commercial work. it is estimated that the number of hours charged at the first, second and third rates are in ratios 4: 2: 1. what percentage of total receipts do receipts from commercial work represent?