Consider a hypothetical economy that grows at a rate of 4% per year. Approximately how many years will it take for this economy to double in size?

Note:the size of an economy is its real GDP.

Hint:to solve this problem, you first want to set up an equation that links the future size of the economy to its current size and the growth rate.

Honestly I have no idea where to start. If anyone has any clue as to solve this I would greatly appreciate some help!