Consider the following statistic.

as of january 2009, the usa has produced 60,000 metric tons of nuclear waste in 60 years of operating 104 nuclear power plants.

on average, how much waste is produced by each plant on a monthly basis?

I just took the test for edgenuity, the answer is A, 0.8 tons per month

We calculate the number of waste produced by each plant on a monthly basis by dividing the given number by the number plant and converting 60 years to months. 
                        r = 60,000 T / (104 x 60 x 12) = 0.8 T/month
Thus, on the average, each of the 104 plants produced approximately 0.8 T/month of waste. 

What? i don’t understand