Anna and Becky have to do a job. Anna would finish the job in 6h if she worked alone. How many hours would it take Becky to do this job, if when working together this works is finished in 4hours?
Anna/s efficiency rate is (1/6) job/h.
In four hours, Anna could finish (4)(1/6)=(2/3) job.
Working with Anna for 4h and completing it, Becky finished
So, Becky requires = 12 h, to finish it all by himself.