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Cogito

Consumer Math · Chapter 3 · Lesson 3

Practice Worksheet

The Rule of 72 and Time

How long until it doubles?

12 problems · about 20 minutes · TEKS M.M.3.C

NameDate

Figure — use these to answer the problems

Each bar is one doubling. The final step adds more than everything before it put together.
Start1
1 doubling2
2 doublings4
3 doublings8
Start1
After 1 doubling2
After 2 doublings4
After 3 doublings8
Figure 1

Warm Up

Straightforward practice. Get the method working first.

5 problems

  1. At 12% a year, how many years until money doubles?

    Answer

  2. Why does starting early matter so much?

    • The final doubling adds more than all the earlier ones combined.
    • Because young savers get better rates.

  3. At 8%, years to double?

    Answer

  4. At 9%, years to double?

    Answer

  5. At 4%, years to double?

    Answer

Build It Up

The same ideas with more to keep track of.

3 problems

  1. $5000 after 3 doublings. Amount in dollars?

    Answer

  2. At 8% over 36 years, how many doublings?

    Answer

  3. At 3% inflation, years until prices double?

    Answer

Stretch Yourself

Mixed problems. Work out what kind of question it is before you start.

4 problems

  1. Match each rate with its doubling time.

    Draw a line from each item on the left to its match on the right.

    • 6% a year
    • 8% a year
    • 12% a year
    • About 12 years
    • About 9 years
    • About 6 years

  2. $2000 after 4 doublings. Amount in dollars?

    Answer

  3. The Doubling

    At 6% a year, how many years until money doubles?

    Answeryears

  4. The Four Doublings

    $10000 after 4 doublings. What is the amount, in dollars?

    Answerdollars