Answer :
A can do 1 job in 10 min. means 1/10 job per min.
B can do 1/15 job per min.
together A&B can do 1/10+1/15 =1/6 job per min.
1/6 job they spend 1 min
1 spend 1x1/(1/6) = 6 min
B can do 1/15 job per min.
together A&B can do 1/10+1/15 =1/6 job per min.
1/6 job they spend 1 min
1 spend 1x1/(1/6) = 6 min