M4.2.2 For the adult
Finding the original amount
A herd grew by 25% and now has 150 goats. How many goats did it have before?
What this chapter teaches
To undo a percentage rise or fall, divide by its multiplier. Taking the same percent off the new amount is a common slip, because the percent was of the old amount. Two changes of the same percent in opposite directions do not cancel.
Three questions to ask
There is no one right wording. Listen for the idea.
- After a rise of 20% an amount is 96. How would you find the amount before?
- Why is taking 20% off 96 not the way back?
- What single number do you multiply by for a rise of 10% followed by a fall of 10%?
After the chapter, the learner can
- Find the original amount after a 20% rise gives 96
- Explain why a 10% rise then a 10% fall is a loss
What they copy into their notebook
To go back from a new amount, divide by the multiplier. Never take the same percent off the new amount. A rise then a fall of 10% gives 0.99.