3. A large mixing tank initially holds 1000 Lof brine. Another brine solution is pumped into the tank at a rate of 10 -. When the solution is well stirred, it is then pumped out at a sowerrate of 7.5 - .If the concentration of salt in the brine entering the tankiso. 25 , derive the differential equation of the salt concentration S() in the tank at any time t . 4. Derive a differential equation for the orthogonal trajectories of the family of curvesy = ce*. Sketch by hand at least 3 membersof the family of curvesy = ce* and at least 3 membersof the family of curvesof its orthogonal trajectoriestogether in 1 graph. 5. Derive a differential equation for the orthogonal trajectoriesof the family of curvesxy = c. 6. Obtain the differential equation associated with the primitive y = Ael* + Be"* + Cet 7. Find the differential equation of a family of circles of fixed radiusp with centers on the x-axis