first you would expect the final answer to be the volume times

the concentration of salt being added

2*10^6 * 10 = 2* 10^7gms

The IVP is dS/dt = Salt in - Salt out

dS/dt = 10gms/litre*60litres/min - S(t)gms/(2*10^6litres)*60(litres/min)

dS/dt = 600 - 3*10^(-5)S to make things easier let a = 3*10^(-5)

dS/dt = 600 - a S

you should obtain S= 2*10^7 - 2*10^7 e^[-3*10^(-5)t]

The limit is 2*10^7 as expected