The average cost of some books is $20.
After 5 new books are added,
it adds another $19 to the total cost.
As a result, the average cost of all the books decreases to $11.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
20 |
11 |
Total value |
20 u |
11 u + 55 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 11 x (1 u + 5)
= 11 u + 55
20 u + 19 = 11 u + 55
20 u - 11 u = 55 - 19
9 u = 36
1 u = 36 ÷ 9 = 4
Number of books at first = 4
Answer(s): 4