When t = 0, i e , before the inflow–outflow of the brine starts,
 y = 250 g × 1000 = 250 kg
Note that the change in y occurs due to the inflow, outflow of the mixture Now the inflow of brine brings salt into the tank at the rate of 5 kg per minute
(as 25 × 200 g = 5 kg) and the outflow of brine takes salt out of the tank at the rate of
25 1000
40
y
y


 =
 kg per minute (as at time t, the salt in the tank is 1000
y
kg)