The average cost of some books is $50.
When another 7 books are added,
the total cost increases by $150.
The average cost becomes $40.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
50 |
40 |
Total value |
50 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 50
= 50 u
Total cost of the books in the end
= 40 x (1 u + 7)
= 40 u + 280
50 u + 150 = 40 u + 280
50 u - 40 u = 280 - 150
10 u = 130
1 u = 130 ÷ 10 = 13
Number of books in the end
= 1 u + 7
= 13 + 7
= 20
Answer(s): 20