FRANK the Engineer was in charge of the construction of a brick retaining wall. Through his extensive supervisory experience, he knew that one of his two bricklayers could build the wall in exactly nine hours and the other in exactly ten hours.
He also knew from previous jobs that if the two bricklayers worked together, ten fewer bricks got laid per hour than would be expected. This of course was due to the fact that both bricklayers enjoyed sharing a yarn and a joke while working  something they couldn't do when working alone.
Since Frank was in a hurry to get the job done, he decided to put both bricklayers on the job, and found it took exactly 5 hours to build the wall. This proved his estimates of their work rates.
How many bricks did the wall contain?
