Question P3–P4–P5–P6 Working backwards
Today’s problem — free all day, every step. Every day has one.
Ravi had some marbles in a jar. He added 20 marbles and removed 8 marbles. Then a magic trick doubled the number of marbles in the jar. There were then 70 marbles. How many marbles were in the jar before he added or removed any?
Answer
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You know how it ended. Undo each step, last one first.
In this one. Work backwards in reverse order, using the opposite operation each time: undo the doubling first, then undo the 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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