The average cost of some books is $43.
When another 7 books are added,
the total cost increases by $220.
The average cost becomes $40.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
43 |
40 |
Total value |
43 u |
40 u + 280 |
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 43
= 43 u
Total cost of the books in the end
= 40 x (1 u + 7)
= 40 u + 280
43 u + 220 = 40 u + 280
43 u - 40 u = 280 - 220
3 u = 60
1 u = 60 ÷ 3 = 20
Number of books in the end
= 1 u + 7
= 20 + 7
= 27
Answer(s): 27