Only use two variables, representing the number of mice and the number of cats.

m = number of mice

c = number of cats

m + c = 60 {mice and cats together equal a total of 60}

3m = c + 20 {three times the number of mice exceeded cats by 20}

m + c = 60 {first original equation, unchanged}

3m - c = 20 {subtracted c from each side of second equation}

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4m = 80 {added the two equations}

m = 20 {divided each side by 4}

m + c = 60 {first equation}

20 + c = 60 {substituted 20, in for m, into first equation}

c = 40 {subtracted 20 from each side}

**40 cats***- Algebra House*
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