Abel can complete a work in 10 days, Ben in 12 days and Carla in 15 days. All of them began the work together, but Abel had to leave after 2 days and Ben 3 days before the completion of the work.
How long did the work last?
Let the work last for x days.
Abel worked for 2 days
Ben worked for (x-3) days
Carla worked for x days
2/10 + (x-3)/12 + x/15 = 1
x/15 + x/12 = (4+5)x/60 = 9x/60 = 1 + 3/12 - 2/10 = 1 + 1/4 - 1/5 = (20 + 5 - 4)/20 = 21/20
x = 21*60/9*20 = 7 days
IMO B