Dating Made Easier

If a sum invested gains 10% each year how long will it be before it has doubled its value?

If an object depreciates in value by 10% each year how long will it take until only half of the original value remains?

Why aren’t these two answers the same?

Is there a rate, used for both gain and depreciation, for which those two answers would actually be the same?

See the notes for teachers and solution 

See the DATING MADE EASIER poster.

This activity is adapted from the NRICH task Dating Made Easier with permission of the University of Cambridge. All rights reserved.