In how many days will the original gang of pipe layers, under the supervision of foreman B, finish the original pipe-laying job?

A gang of pipe layers working under the supervision of foreman A can finish a pipe-laying job in 20 days. The same gang of pipe layers working without the supervision of a foreman can finish the same pipe-laying job in 25 days. The project engineer, desiring to finish the job in 20 days, employed the gang of pipe layers under the supervision of foreman A. However, at the end of the fifth working day, foreman A quit the job. In order to finish the job on the original target period of 20 working days, the project engineer employed two additional pipe layers at the start of the 6th working day. After the enlarged gang had worked 2 days without the supervision of a foreman, the project engineer decided to finish the job ahead of his original schedule. He employed foreman B to supervise the enlarged gang at the start of the 8th working day. The enlarged gang of pipe layers, under the supervision of foreman B finished the job at the end of the 18th working day, 2 days ahead of the schedule.

In how many days will the original gang of pipe layers, under the supervision of foreman B, finish the original pipe-laying job?