Answer:
worker A: 12 hours to do a job, i.e. 1/12 work per hour
worker B: 6 hours to do a job, i.e. 1/6 job per hour
between them 2 per hour they do 1/12 + 1/6 = (1+2)/12 = 3/12=1/4 of work per hour
they do 1/4 in 1 hour
they make 1/4+1/4=2/4 = 1/2 in 2h
they make 1/2+1/4 =3/4 in 3 hours
they do 3/4+1/4 = 4/4 i.e. the whole work in 4 hours
it takes them 4 hours to do the job