A+B-B-C=40-30= 10, which means A-C= 10.

Because A+C=50

Adding them together, we get 2A=60, A=30, C=20, and B= 10.

Or directly add three formulas to get 2(A+B+C)= 120, then A+B+C=60.

The second question: buy three bottles and get one free, which is three times as much as buying gifts. My mother gets 20 bottles of gifts, which is to buy 20×3=60 bottles.

60+20=80 bottles

Hope to adopt, thank you.