The average cost of some books is $10.
After 3 new books are added,
it adds another $90 to the total cost.
As a result, the average cost of all the books increases to $16.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
10 |
16 |
Total value |
10 u |
16 u + 48 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 10 x 1 u
= 10 u
Total cost of books in the end
= 16 x (1 u + 3)
= 16 u + 48
16 u + 48 = 10 u + 90
16 u - 10 u = 90 - 48
6 u = 42
1 u = 42 ÷ 6 = 7
Number of books at first
= 1 u
= 7
Answer(s): 7