Answer:
6
Explanation:
The work involved in making the poster is the product of students and hours:
work = students × hours = (9 students)(10 hours) = 90 student·hours
__
needed
If hours are reduced to 6, the necessary number of students is ...
90 student·hours = (6 hours)(students)
(90 student·hours)/(6 hours) = 15 students . . . . . . divide by 6 hours
additional
If 15 are needed, then the number that must be added is ...
15 -9 = 6 . . . . students need to be added