Question P3–P4–P5–P6 Working backwards
Today’s problem — free all day, every step. Every day has one.
There were some passengers in a cable car. At the first station, 15 passengers boarded and 6 got off. At the second station, the number of passengers doubled. There were then 50 passengers. How many passengers were in the cable car before the first station?
Answer
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You know how it ended. Undo each step, last one first.
In this one. Work backwards in reverse order: undo the doubling first, then undo the first station's net gain.
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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