Through Math for America, I am part of an on-going collaboration with the New York Times Learning Network. My latest contribution, a Test Yourself quiz-question, can be found here:
This question relates to mutual funds and investment growth: how much would $1,000 be worth after 35 years?
I like using the Rule of 72 for this kind of estimation. 72/6 = 12, meaning that it will take approximately 12 years to double; in 35 years it will double about three times.
Yes, that’s a great approach, and a valuable rule-of-thumb. I encourage you to comment on the Learning Network directly, so students will see it with the problem itself.
Also, figuring out why the rule works, and generalizing other such rules, is a great opportunity to abstract.
There’s a limit as how much can go in the “solution”, so I opted to focus on narrating the repeated multiplication aspect of the problem.