The answer is that she needs 250 grams.
There is a need to calculate all the mentioned values in the similar units. In case, if one is choosing kilograms, then,
1 kilogram is equal to 1000 grams.
So, it can be written that Dr. O is having 750 grams of potting soil and she needs 1000 gm. In order to calculate the remaining amount, there is a need to subtract the mentioned values, that is,
1000 gm - 750 gm = 250 gm
Thus, finally, it can be written that Dr. O requires 250 grams of potting soil.
