The average cost of some books is $78.
When another 8 books are added,
the total cost increases by $120.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 8 |
Value |
78 |
60 |
Total value |
78 u |
60 u + 480 |
Number of books at first = 1 u
Number of books in the end = 1 u + 8
Total cost of books at first
= 1 u x 78
= 78 u
Total cost of the books in the end
= 60 x (1 u + 8)
= 60 u + 480
78 u + 120 = 60 u + 480
78 u - 60 u = 480 - 120
18 u = 360
1 u = 360 ÷ 18 = 20
Number of books in the end
= 1 u + 8
= 20 + 8
= 28
Answer(s): 28