The 3 inside angles of a triangle need to add up to 180 degrees.
You have two angles of 104 and 36.
A = 180 - 104 - 36 - 40
The answer is 40
At any time 
(min), the volume of solution in the tank is
If 
is the amount of salt in the tank at any time , then the solution has a concentration of 
.
The net rate of change of the amount of salt in the solution, 
, is the difference between the amount flowing in and the amount getting pumped out:
Dropping the units and simplifying, we get the linear ODE
Multiplying both sides by 
allows us to identify the left side as a derivative of a product:
Integrate and divide both sides by to get
The tanks starts off with 30 lb of salt, so 
and we can solve for 
to get a particular solution of
