Let "x" be the amount the bottle can hold.
Then, the difference between being (1/4) full and (2/3) full = 500ml.
In math terms, we can write (2/3)x - (1/4)x = 500
Or , put anther way, (2/3 - 1/4)x = 500
So we have (5/12)x = 500
Multiply by (12/5) on both sides
So, we have x = 500*(12/5) = 1200ml
Check to see that this makes sense!! 2/3 of 1200 = 800 1/4 of 1200 = 300 800 - 300 = 500
And there you go......
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