Question

Workers A and B working together can finish a job in 8 hrs.If they work together for 6 hrs.,after which worker A leaves,then worker B needs 9 more hrs to finish the job.How long does it take worker A to do the job alone?

Answer 1

1/A + 1/B = 1/8
... 15/B = 15/8 - 15/A

6/A + 15/B = 1
... 6/A - 15/A + 15/8 = 8/8
... -9/A = -7/8
... -72 = -7A

Answer 2

or, equivalently,

1/A + 1/B = 1/8
6/8 + 9/B = 1