The average cost of some books is $26.
After 6 new books are added,
it adds another $12 to the total cost.
As a result, the average cost of all the books decreases to $18.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
26 |
18 |
Total value |
26 u |
18 u + 108 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 26 x 1 u
= 26 u
Total cost of books in the end
= 18 x (1 u + 6)
= 18 u + 108
26 u + 12 = 18 u + 108
26 u - 18 u = 108 - 12
8 u = 96
1 u = 96 ÷ 8 = 12
Number of books at first = 12
Answer(s): 12