ACT Exam Prep (Math and English)

Answer: D - To calculate this answer mentally, use the Rule of 72.

Divide 72 by the rate of growth. In this example, Ron’s money would double in 8 years (72/9). The rule of 72 is a quick mental calculation tool that provides an estimate of how long it would take an investment to grow double.


Key Takeaway: For a few days, practice this every time you see a growth rate in the news paper or at work.