The average cost of some books is $30.
After 6 new books are added,
it adds another $20 to the total cost.
As a result, the average cost of all the books decreases to $26.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
30 |
26 |
Total value |
30 u |
26 u + 156 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 26 x (1 u + 6)
= 26 u + 156
30 u + 20 = 26 u + 156
30 u - 26 u = 156 - 20
4 u = 136
1 u = 136 ÷ 4 = 34
Number of books at first = 34
Answer(s): 34