The average cost of some books is $10.
After 4 new books are added,
it adds another $100 to the total cost.
As a result, the average cost of all the books increases to $15.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 4 |
Value |
10 |
15 |
Total value |
10 u |
15 u + 60 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 10 x 1 u
= 10 u
Total cost of books in the end
= 15 x (1 u + 4)
= 15 u + 60
15 u + 60 = 10 u + 100
15 u - 10 u = 100 - 60
5 u = 40
1 u = 40 ÷ 5 = 8
Number of books at first
= 1 u
= 8
Answer(s): 8