Wildfires in the united states covered three and forty-two hundredths million acres one year. the second year, they covered eight and seventy-one hundredths million acres. the three-year total of wildfire acres burned was twenty-one and forty-sixdths acres. how many acres were burned in year three?

Everything is in million acres, so we can just drop that while we work it out but just remember we have to add it back on at the end of the problem.

First year 3.42
Second year 8.71
Third year ???

Total 21.46

So...

 3.42 + 8.71  + ? = 21.46
12.13 + ? = 21.46
? = 21.46 - 12.13
? = 9.33

The 3rd year 9.33 million acres were burned

*Note I put a ? for the unknown.  I could have used x or y or anything else.

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