A deposit can be filled by two keys. One poured 140 liters in 8 minutes and the other 130 liters in 7 minutes. How long it will take to fill the tank, drain being empty and closed?, if the two keys are open at the same time, knowing that its capacity is 400 liters?

Its a matter of calculating the two speeds of filling and then add them together.

speed1 = 140 liters / 8 minutes = 17.5 liters per minute

speed2 = 130 liters / 7 minutes = 18.57 liters per minute

So we'll open both keys with speed1 and speed2, x amount of minutes, and fill 400 liters, if we write an equation of these statement that is:

17.5x + 18.57x = 400

solve for x!

36.07x = 400

x = 11.09

therefore it will take 11.09 minutes to fill the 400 liters tank with both keys open.