The average cost of some books is $26.
After 3 new books are added,
it adds another $10 to the total cost.
As a result, the average cost of all the books decreases to $22.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
26 |
22 |
Total value |
26 u |
22 u + 66 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 26 x 1 u
= 26 u
Total cost of books in the end
= 22 x (1 u + 3)
= 22 u + 66
26 u + 10 = 22 u + 66
26 u - 22 u = 66 - 10
4 u = 56
1 u = 56 ÷ 4 = 14
Number of books at first = 14
Answer(s): 14