I don't know if you found the answer for this since it's been 5 hours since you posted this but I hope this helps.
So to figure out the amount of mustard on each hot dog you would divide 8.5 ounces by 25. So... 8.5/25 is .34. So he puts .34 ounces of mustard on each dog. So if he eats 7 hot dogs he uses 2.38 ounces of mustard. (7 x .34). Then you just subtract that amount from 8.5, so 8.5 - 2.38 = 6.12. So there is 6.2 ounces of mustard left after he eats 7 hot dogs. Again I hope this helps since it's been so long since the question was asked :) (he also has enough mustard left in the bottle to eat 18 more hot dogs with the same amount of mustard on them)
