tank contains 1000L of water with 15kg of salt dissolved in it. Pure water enters tank at 10L/min and the mixture leaves at the same rate. How much salt is in the tank after t minutes?

Work:

let y(t)=mass of salt in water

y(0)=15kg

dy/dt = rate in - rate out

rate in = 0 because no salt is entering

rate out =

This becomes a seperatable equation so I get solving for y I get

The answer given is