Hi Andreia,

this is definitely a case where you can apply the rule of 72 to get a rounded answer:

• Step 1: find the amount of time the initial capital would double. Dividing 72 by 4, you get 18
• Step 2: find how many times the amount would double in 54 years. As 54/18=3, the amount would double 3 times
• Step 3: find the actual value. As $100k would double 3 times, you would get $800k:
  • Doubling the first time would go to $200k
  • Doubling the second time would go to $400k
  • Doubling the third time would go to $800k

The actual answer is $831k thus you would be pretty close.

In this particular case you cannot use shortcuts as the simple interest or the Taylor series, as the period of time is too long.

Hope this helps,

Francesco