Bunuel wrote:
At an elementary school, the students in third grade, fourth grade, and fifth grade run an average of 12, 15, and 10 minutes per day, respectively. There are twice as many third graders as fourth graders, and twice as many fourth graders as fifth graders. What is the average number of minutes run per day by these students?
A. 12
B. 37/3
C. 88/7
D. 13
E. 14
The Q is, Avg #of min/day by all students=
Total time taken by students / total #of studentsLet's say, 3rd grade student= x
4th grade student= y
5th grade student= z
As Q says, Twice as many 3rd graders as 4th graders: x= 2y
Twice as many 4th graders as 5th graders: y=2z
We can also say, x= 2*2z= 4z
Total # of students= x+y+z= 4z+2z+z= 7z
Total time taken by all students= 12x+15y+10z= 12*4z+15*2z+10*z = 88z
Going back to the formula: 88z/7z =
88/7 is our ans.