Solve the Problem $\frac { d y } { d t } = 18 - \frac { 6 } { 120 + 3 t } y$

Multiple Choice

Solve the problem.
-A tank initially contains 120 gal of brine in which 40 lb of salt are dissolved. A brine containing 2 lb/gal of salt runs into the tank at the rate of 9 gal/min. The mixture is kept uniform by stirring and flows out of the tank at
The rate of 6 gal/min. Write, in standard form, the differential equation that models the mixing process.

A) $\frac { d y } { d t } = 18 - \frac { 6 } { 120 + 3 t } y$
B) $\frac { d y } { d t } + \frac { 9 } { 120 + 3 t } y = 12$
C) $\frac { d y } { d t } + \frac { 6 } { 120 + 3 t } y = 18$
D) $\frac { d y } { d t } + \frac { 6 } { 120 - 3 t } y = 18$