She returned the bottle of 250 milliliter in lithium stock where another 175 milliliter of lithium was already present in the stock.
Quantity of lithium before return = 175 milliliter.
Quantity of lithium that she returned = 250 milliliter.
Total stock would be now = 175 + 250 = 425 milliliter.
So, there was 425 milliliter of lithium before she used any of it.
Hence, 425 milliliter is the final answer.
