The average cost of some books is $26.
After 2 new books are added,
it adds another $12 to the total cost.
As a result, the average cost of all the books decreases to $21.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
26 |
21 |
Total value |
26 u |
21 u + 42 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 26 x 1 u
= 26 u
Total cost of books in the end
= 21 x (1 u + 2)
= 21 u + 42
26 u + 12 = 21 u + 42
26 u - 21 u = 42 - 12
5 u = 30
1 u = 30 ÷ 5 = 6
Number of books at first = 6
Answer(s): 6