20% of 100 would be 20; so if we add on 20 to 100 we get:
120 grams to make a 10 cup pot of coffee.
Now, we need to convert 120 grams for 10 cups, into 15 cups.
So we essentially need to add on 5 more cups of coffee: So we will take half of the amount it took to make 10, and add it.
(120g/2 + 120g ) = 15 cup brew.
(60g + 120g) = 15 cup brew
180g = 15 cup brew.
Therefore, he will need 180g to make a 15-cup pit if Arabica Coffee.
I Hope this Helps!