The average cost of some books is $74.
When another 4 books are added,
the total cost increases by $240.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
74 |
70 |
Total value |
74 u |
70 u + 280 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 1 u x 74
= 74 u
Total cost of the books in the end
= 70 x (1 u + 4)
= 70 u + 280
74 u + 240 = 70 u + 280
74 u - 70 u = 280 - 240
4 u = 40
1 u = 40 ÷ 4 = 10
Number of books in the end
= 1 u + 4
= 10 + 4
= 14
Answer(s): 14