Two containers a and b begin with equal volumes of liquid. 120 ml is then poured from a to b. container b now contains 4 times as much liquid as a. find the volume of liquid left in container a at the end.

Define A  and B:

Let A be the volume of the liquid in container A at first.
Let B be the volume of the liquid in container B at first.


Construct equation :

2 containers have the same liquid.
⇒ A = B

120 ml is then poured from A to B. Container B now contains 4 times as much liquid as A.
⇒ 4(A - 120) = (B + 120)


Solve A and B :

A = B                        (1) 
4(A - 120) = (B + 120) (2)

Sub (1) into (2) :
4(B - 120) = (B + 120)
4B - 480 = B + 120
3B = 480 + 120
3B = 600
B = 200 


Find A and B :

B = 200 ml
A = B = 200 ml

Container A at the end = 200 - 120 = 80ml


Container A has 80m left.

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