Answer:
It will take them 6.35 minutes
Explanation:
The question can be solved when you know how much of the job can be done in minutes.
Therefore if Dan can do the job in 15 minutes he will be to sweep 1/15 of the porch in 1 minutes. Alberto can do the same job in 11 minutes, in 1 minutes he will do 1/11 of the job.
let
a = combined time together
1/15 + 1/11 = 1/a
(11 + 15)/165 = 1/a
26/165 = 1/a
cross multiply
26a = 165
divide both sides by 26
a = 165/26
a = 6.34615384615
a ≈ 6.35 minutes
It will take them 6.35 minutes