Straight line depreciation used with a percentage?

we were asked to compose an income statement and balance sheet as an accounting assessment! but I thought straight line depreciation was getting the original cost of the assest - the scrap value divided by the number of years it was estimated to be used for. but one example of what I want to know is : fixtures and fittings were �500,000 and accumulation is 242,000 and at the end of the question it says that depreciation is 20% straight line? does anyone know how to calculate this???and then again for motor vehicles it says the original cost was 235,000 accuumulated depreciation was 135,000 the it says motor vehicles costing 35,000 with accumulated depreciation of 15,000 was sold for 17,000!how do u calculated the 20?% straight line on the motor vehicles???? Thanks sooooo much :)