A heater can raise the temperature of a gallon of water 10 degrees in 8 minutes. Another can raise the temperature of a gallon of water 10 degrees in 5 minutes. After the first heater had been operating 1 minute, the second heater was turned on, and both were operated. How long did it take to raise the temperature of the water 10 degrees?

let x - time for gallon 1

    y  - time for gallon 2

8 - 1 = 7 minutes needed for gallon 1

7/8 = x/5

8x = 35

x = 35/8

x = 4.37 minutes x 60 secs = 4 minutes and 22 secs.