Question P5–P6 Working backwards
Today’s problem — free all day, every step. Every day has one.
A shuttle bus left the airport carrying some tourists. At the first station, of the tourists in it alighted and 5 tourists boarded it. At the second station, of the tourists in it alighted and 11 tourists boarded the bus. When it left the second station, there were 30 tourists in it. How many tourists were there in the bus when it left the airport?
Answer
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You know how it ended. Undo each step, last one first.
In this one. Work backwards from the end, undoing each event in reverse order — which means at each stop you undo the boarding before the alighting, because forwards the alighting happened first.
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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