Let's look at how to figure out the amount after 18 years.

At the end of 6 years, the $5,000 has doubled to $10,000.

At the end of 6 *more* years -- or 12 years total -- that $10,000 will have doubled to $20,000.

At the end of 6 *more* years -- or 18 years total -- that $20,000 will have doubled to $40,000.

So, after 18 years, there will be $40,000.

You can figure out the amount after 42 years. Just remember that 6 x 7 = 42, so there will be a total of 7 doublings. The 18 years took care of the first three, so you need to figure out 4 more doublings.

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