(4)
Here, the equation (4) gives the time t at which the salt in tank is y kg Step 4 Since 
40
t
e
−
 is always positive, from (3), we conclude that y > 200 at all times
Thus, the minimum amount of salt content in the tank is 200 kg Also, from (4), we conclude that t > 0 if and only if 0 < y – 200 < 50 i e