Atank is being filled with water using an old
pump that slows down as it runs. the table below
gives the rate at which the pump pumps at 10
minute intervals. if the tank is initially empty,
estimate how much water is in the tank after 90
minutes.

Found a similar problem with the given table. 

Elapsed time (Minutes)   0    10   20   30    40    50   60   70   80   90
Rate (gallons / minute)   42   40  38   35    35    32   28    20   19   10

Because the interval is every 10 minutes, we will use 10 and multiply it to each rate and get the sum total.

42*10 + 40*10 + 38*10 + 35*10 + 35*10 + 32*10 + 28*10 + 20*10 + 19*10 + 10*10
420 + 400 + 380 + 350 + 350 + 320 + 280 + 200 + 190 + 100
2,990

There will be a total of 2,990 gallons in the tank after 90 minutes. 

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