Answer:
47.25 pounds
Step-by-step explanation:

<u>First, we determine the Rate In</u>
Rate In=(concentration of salt in inflow)(input rate of brine)

Change In Volume of the tank, 
Therefore, after t minutes, the volume of fluid in the tank will be: 100+2t
<u>Rate Out</u>
Rate Out=(concentration of salt in outflow)(output rate of brine)

Therefore:

This is a linear differential equation in standard form, therefore the integrating factor:

Multiplying the DE by the integrating factor, we have:

Initially, 20 pounds of salt was dissolved in the tank, therefore: A(0)=20

Therefore, the amount of salt in the tank at any time t is:

After 15 minutes, the amount of salt in the tank is:
