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Let the total amount of salt in two barrels be y 1 and y2, respectively. The unit is L, the unit of T is the second record (personal habit problem), and S is the required quantity.
dy 1 =(-y 1/500)*(40/60)；
dy2 =-dy 1-(y2/(500+20 * t/60))*(20/60)；
y 1(0)= 500 * 10.5；
y2(0)= 500 * 10.5；
s = int((y2/(500+20 * t/60))*(20/60)，t，0，t)；
The answer is s = (3937500 * exp (-t/750)-19687500)/(t+1500)+10500;
Note that the unit is seconds.