How many mice were there?

Mrs. Fernsler and Mrs. Anderson were raising African spotted mice.

The mice were unusual in that every two months the female had exactly two babies - a male and a female.

When these babies were two months old, they had their first babies and continued to have them every two months after that.

Mrs. Fernsler and Mrs. Anderson began with one pair of spotted mice on January 1, 2001, and they decided to give away all that they had on January 2, 2003, just after more babies were born. Since none of the mice had died, they had quite a few.

How many mice were there by January 2, 2003?

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We drew a family tree of mice up to 32 mice, and then we saw a pattern which was that the number of mice doubled every 2 months.  Ben S.

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adapted from Make It Simpler:A practical guide to problem solving in mathematics,  Carol Meyer and Tom Shallee, Addison Wesley, 1983