The average cost of some books is $76.
When another 7 books are added,
the total cost increases by $180.
The average cost becomes $60.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
76 |
60 |
Total value |
76 u |
60 u + 420 |
Number of books at first = 1 u
Number of books in the end = 1 u + 7
Total cost of books at first
= 1 u x 76
= 76 u
Total cost of the books in the end
= 60 x (1 u + 7)
= 60 u + 420
76 u + 180 = 60 u + 420
76 u - 60 u = 420 - 180
16 u = 240
1 u = 240 ÷ 16 = 15
Number of books in the end
= 1 u + 7
= 15 + 7
= 22
Answer(s): 22