Paraphrasing
Party ate 401 lb of food. No one at more than 2.5lb what is the minimal number that have attended.
401/2.5. 160.4. It must be round up because you need at least 161 people to eat 401lb.
What if the problem changed to
Party has 401 lb of food. No one is going to eat more than 2.5lb. How many people can you invite?
In this case dont you have to round down to 160 because you dont want to run out of food?