A child can do a piece of work 15 hours slower than a woman. The child works for 18 hours on the job and then the woman takes charge for 6 hours. In this manner `3/5,` of the work can be completed. To complete the job now, how much time will the woman take? a. `12\ hou r s` b. `18\ hou r s` c. `24\ hou r s` d. `30\ hou r s`