So whatever happened does not matter, They are only asking the volume of liquid left in A at the end. Therefore :
Calculation :
B + 120 = 4A
B = 4A - 120 ---------- 1
A - 120 = B ------------ 2
Subtract 2 from 1
B - B = 4A - 120 - A - 120
0 = 3A - 240
240 = 3A
240 ÷ 3 = A
8 = A
Answer :
A = 8
