Let c be the concentration. Then c changes according to:
The amount of salt entering the tank is pounds per minute.
The amount of salt leaving the tank is c*4 pounds per minute.
So you solve the differential equation and apply the initial condition c(0)=1 pound per gallon to evaluate the constant of integration, then set c(t)=0.7 and solve for t.
Post again in this thread if you are still having trouble.