Question

If 2 workers takes 15 hours to make 3 fireplaces, how long 5 workers takes to make 4 fireplaces ?

Answer 1

2 workers takes 15 hours to make 3 fireplaces

2 workers takes 5 hours to make 1 fireplace
2 workers takes 20 hours to make 4 fireplaces
1 worker takes 40 hours to make 4 fireplaces
5 workers takes 8 hours to make 4 fireplaces

or

it takes 30 work-hours to make 3 fireplaces
it takes 10 work-hours to make 1 fireplace
it takes 40 work-hours to make 4 fireplaces
so it would take 5 workers 8 hours to make the 4 fireplaces.