When he was measuring herbs on his scale, neville place 1/2 gram weight, then 1/5 gram weight, and finally 1/8 gram before the scale was balanced. how much did the herbs weigh?

So basically the question just wants you to add fractions so we set up the problem like this
1/2+1/5+1/8=weight of herbs=x (x signifies an unknown number)

so we have to get the bottom numbers the same before we can add them so we have to find the smallest number that they all can multiply into
so
       2=2
       5=5
       8=2*2*2
       so the smallest number that that will go into all three of the numbers is
     2*2*2*5 =40
so the bottom number will be 40
so 1/2 is equal to 20/40
1/5=8/40
1/8=5/40
so 20/40+8/40+5/40=33/40
answer is 33/40 grams
so