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)
Remainder => 750ml + 750ml
Remainder => 750ml x 2
You found the remainder, what is next? You add up what Joash had drunk with the remainder, right?
Bought => 500ml + 1500ml
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.