A, B and C can do job in 9, 12 and 36 days respectively if they worked alone. A leaves after they have worked together for 3 days. In how many days can B and C do the rest of the job?

A, B, C working together units is LCM of 9, 12, 36= 36 units

A can work $$36\div9$$= 4 units in a day.

B can work $$36\div12$$= 3 units in a day.

C can work $$36\div36$$= 1 units in a day.

A, B, C 3 days work= $$4\times3+3\times3+1\times3$$= 24 units.

Remaining work is to be done by B & C= (36-24)units= 12 units.

Total days taken by B & C together to complete the work after A leaves= $$12\div(3+1)$$= 3 days.

Therefore, Option A is correct.