To answer this question, let us start by making a representation of the situation
We are asked to calculate in how much time the tank will be filled, note that 3 liters of water are falling into the tank every second so we have the next sequence of states
* After one second, there are 3 liters in the tank;
*After two seconds, there are 6 liters in the tank;
* After 3 seconds, there are 9 liters on the tank
And so on, so to find how many seconds it will take to fill the tank we must find haw many times i can put 3 liters of water in the tank, that is, we have to calculate:
[tex]How\text{ many times I can put 3 liters in the tank= }\frac{54}{3}=18=\text{ It takes 18 seconds }[/tex]We conclude that to fill a tank with capacity 54 liters putting water at rate of 3 liters per second it will take 18 seconds.