Question P6 Working backwards
Today’s problem — free all day, every step. Every day has one.
A baker sold a wedding cake at a 15% profit for $115. What was the cost price of the cake?
Answer
The cost price was $100.
You know how it ended. Undo each step, last one first.
In this one. A profit percentage is always taken of the cost price, so the cost price is the 100% — the selling price is 115%, and $115 must be matched to 115% before you step down to 1% and back up to 100%.
Also uses: Units and parts
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%.
Worked examples of all 33 patterns and skills, in a booklet you can print.