The average cost of some books is $77.
When another 9 books are added,
the total cost increases by $490.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 9 |
Value |
77 |
70 |
Total value |
77 u |
70 u + 630 |
Number of books at first = 1 u
Number of books in the end = 1 u + 9
Total cost of books at first
= 1 u x 77
= 77 u
Total cost of the books in the end
= 70 x (1 u + 9)
= 70 u + 630
77 u + 490 = 70 u + 630
77 u - 70 u = 630 - 490
7 u = 140
1 u = 140 ÷ 7 = 20
Number of books in the end
= 1 u + 9
= 20 + 9
= 29
Answer(s): 29