-- It takes 10 workers 3 days to build 1 house.
so
-- It takes 10 workers 1 day to build 1/3 of a house.
so
-- It takes 1 worker 1 day to build 1/30 of a house.
You got 15 workers and you need 4 houses ?
Well, as we just calculated . . .
-- It takes 1 worker 1 day to build 1/30 of a house.
so
-- It takes 15 workers 1 day to build 15/30 = 1/2 of a house.
so
-- It takes 15 workers 2 days to build 1 whole house.
so
-- It takes 15 workers 8 days to build 4 houses.