Maths · Rung 26 of 30

Change factor and interest

Going up 15% is multiplying by 1.15, and going down 15% is multiplying by 0.85: that number is the change factor. Two changes in a row multiply, which is why going up 10% and then down 10% does not get you back to the start. Compound interest is the same factor applied every year.

Where this sits

This is where the ladder starts: nothing is needed before it.

Exercises from this rung

Generated by the app, from easiest to hardest

With a 25% discount you pay 90 €. What did it cost before?
Answer 120
Why You pay 75%: price × 0.75 = 90 €, so price = 90 € ÷ 0.75 = 120 €.
easier
With a 10% discount you pay 54 €. What did it cost before?
Answer 60
Why You pay 90%: price × 0.9 = 54 €, so price = 54 € ÷ 0.9 = 60 €.
in between
A price of 150 € goes down 10% and then goes down 10%. What is the final price?
Answer 121.5
Why 150 € × 0.9 × 0.9 = 121.50 €. Percentages do not add: the second applies to the price already changed.
harder