The average cost of some books is $27.
After 6 new books are added,
it adds another $10 to the total cost.
As a result, the average cost of all the books decreases to $19.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
27 |
19 |
Total value |
27 u |
19 u + 114 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 27 x 1 u
= 27 u
Total cost of books in the end
= 19 x (1 u + 6)
= 19 u + 114
27 u + 10 = 19 u + 114
27 u - 19 u = 114 - 10
8 u = 104
1 u = 104 ÷ 8 = 13
Number of books at first = 13
Answer(s): 13