The average cost of some books is $80.
When another 7 books are added,
the total cost increases by $320.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
80 |
70 |
Total value |
80 u |
70 u + 490 |
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 80
= 80 u
Total cost of the books in the end
= 70 x (1 u + 7)
= 70 u + 490
80 u + 320 = 70 u + 490
80 u - 70 u = 490 - 320
10 u = 170
1 u = 170 ÷ 10 = 17
Number of books in the end
= 1 u + 7
= 17 + 7
= 24
Answer(s): 24