Ben can prepare a company report in 3 hours. This means that the time rate with which Ben can prepare the report is 1/3
Shane can prepare a report in 4.2 hours. This means that the time rate with which Shane can prepare the report is 1/4.2
Let the time at which they would prepare the report if they work simultaneaosly be t. Then, their combined time rate is 1/t
Thus, it means that
1/3 + 1/4.2 = 1/t
(4.2 + 3)/12.6 = 1/t
7.2/12.6 = 1/t
By cross multiplying, it becomes
7.2t = 12.6
t = 12.6/7.2
t = 1.75
If they work together, they will prepare a report in 1.75 hours. Thus, the time needed to prepare eight reports is
1.75 * 8 = 14 hours
It will take them 14 hours to prepare 8 reports