The average cost of some books is $54.
When another 7 books are added,
the total cost increases by $270.
The average cost becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
54 |
50 |
Total value |
54 u |
50 u + 350 |
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 54
= 54 u
Total cost of the books in the end
= 50 x (1 u + 7)
= 50 u + 350
54 u + 270 = 50 u + 350
54 u - 50 u = 350 - 270
4 u = 80
1 u = 80 ÷ 4 = 20
Number of books in the end
= 1 u + 7
= 20 + 7
= 27
Answer(s): 27