a large tank is filled to capacity with 500 gallons of pure water. brine containing 2 pounds of salt per gallon is pumped into the tank at a rate of 5 gal/min. the well mixed solution is pumped out at 10 gal/min. find the number A(t) of pounds of salt in the tank at time t.

ok so I know dA/dt = rate in - rate out.

so dA/dt = (2 lbs/gal)(5 gal/min) - ()???

ok im stuck with how to figure out the rate out?

my book is saying the rate out = 2A/(100-t)??? i dont see how the book came up with that answer...any thoughts?

thanks in advance....