I think the amount of cups needed are 16.8 because if you set up the problem (but simplify the fraction into a decimal first) you set it up as 10.5/5=x/8 then you cross multiply 10.5*8(times) and get 84, last you divide by 8 and you get 16.8. I don't know what that is as a mixed number but I'm sure you can figure it out.