Dr. o is playing in her garden again. she needs 1 kg of potting soil for her plants. she has 750g. how much more does she need?

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.  

sorry, but you do not have posted the diagram.