Algebra
posted by Bob .
"If 3 people can do a job in 5 days, how many days will be needed if 4 people do the same job?"

if it took 3 people 1 day, each person would be doing 1/3 job/day
It takes 5 days, so a person does 1/15 job/day
4 people can therefore do 4/15 job/day
a whole job will therefore take 15/4 days