Working backwards,
750 ml x 2 = 1500 ml
1500 ml + 500 ml = 2000 ml = 2 l

Ans : 2 l.

Understand this: if Joash drank 500ml of the guava juice. 

Can you find the remainder? (What is left after he drank 500ml of the guava juice)

Yes, right? 

Remainder => 750ml + 750ml 

                         = 1500ml 

Remainder => 750ml x 2

                        = 1500ml

You found the remainder, what is next? You add up what Joash had drunk with the remainder, right? 

Bought => 500ml + 1500ml

                = 2000ml

Now we have to change it to litres.

2000ml/1000 = 2 litres

But why divide it by 1000? Litres is 1000ml, right? 

So two 2000ml is equal to 2 litres. 

So that is question solved.

Since he poured equally , each jug contains 750ml

750ml +750ml = 1500 ml ( this is for both jars/ the volume he divided into 2 jars)

Initial volume = 1500+500

=2000ml 

1l =1000ml 

2000/ 1000 = 2litre