Quick
Answer:
Between
9 and 11 hours per week, depending
on your approach to your problem. 

Complete
Solution:
There are many ways to do this problem.
The answer can only be approximated
because of the way the data are reported.
If you assume that the table
shows data for 100 students, then the 7% who
worked from one to five hours would correspond to seven
students who worked one to five hours. To find an average
(mean), you need to estimate the total number of hours
worked. Consider the seven students who worked from one
to five hours. Taken together, the
least time they could have worked is seven hours.
The greatest they could have worked is 35 hours.
(The actual number is probably somewhere in between these
values.) The first chart shows the least number of hours
the 100 students could have worked, while the second chart
shows the greatest number of hours they could have worked.
Least
Number of Hours 
Number
of Students^{1} 
Least
Number of Total Hours 
0 
36 
0 
1 
7 
7 
6 
9 
54 
11 
11 
121 
16 
17 
272 
21 
21 
441 
Total 
100 
895 
Greatest
Number of Hours 
Number
of Students^{1} 
Least
Number of Total Hours 
0 
36 
0 
5 
7 
35 
10 
9 
90 
15 
11 
165 
20 
17 
340 
25^{1} 
21 
525 
Total 
100 
1155 
^{1}estimated
To find the lower value for
the average number of hours worked per week,
divide the total number of hours by the number of students:
895 ÷ 100 = 8.95. The larger value for the average
workweek can be foud in the same way: 1155 ÷ 100
= 11.55. Using this approach, an estimate for the average
for a high school senior is between 8.5 and 12.5 hours.
Another way to estimate
the average uses an average for
the hours worked in each category.
Smallest
Number of Hours 
Largest
Number of Hours 
Average
Number of Hours 
Number
of Students^{1} 
Total
Hours Worked 
0 
0 
0 
36 
0 
1 
5 
3 
7 
21 
6 
10 
8 
9 
72 
11 
15 
13 
11 
143 
16 
20 
18 
17 
306 
21 
25^{1} 
23 
21 
483 


Total 
100 
1025 
^{1}estimated
To find a value for the average, divide
the total number of hours by the number of students:
1025 ÷ 100 = 10.25. This method results in an average
of about 10 and 1/4 hours. 
