First, you need to add Substances A & B together, getting your total amount of the mixture.
The problem is, you can't add numbers with different denominators, so you have to multiply 1/2 by 2.
1/2 × 2 = 2/4
So now, you add 3/4 to your 2/4 to get your mixture total.
3/4 + 2/4 = 5/4
You'll want to leave it in an improper fraction for now. It's much easier to calculate than with a mixed number.
Now, you could do this last step a few different ways, but here is one way.
You know that you need 1/8 of a gram for your experiment, and you have 5/4 of a gram TOTAL. One way to go about it is to simply change your number for the total mixture into 8ths.
5/4 × 2 = 10/8
This might not work for all equations, but in this case, if you need 1/8 of a gram for your experiment, you could simply tell that you have 10/8 of a gram of total mixture, equaling the possibility of 10 total experiments.