Question P4–P5–P6 Working backwards
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A mechanic had a tank of oil. He used half of it to service cars in the morning. In the afternoon he used a further 3.5 litres and was left with 2.5 litres. How many litres of oil did the tank hold at first?
Answer
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You know how it ended. Undo each step, last one first.
In this one. Work backwards: rebuild the half kept after the morning from the afternoon use and the amount left, then double that half to recover the whole tank.
The question gives the final amount after a sequence of changes and asks what the amount was 'at first' or 'to begin with'.
Children undo the steps in the same order they happened rather than in reverse, or forget that a percentage profit is added to the cost price, so the final price is 120% of it, not 100%.
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