The average cost of some books is $10.
After 4 new books are added,
it adds another $100 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 + 4 |
Value |
10 |
16 |
Total value |
10 u |
16 u + 64 |
Number of books at first = 1 u
Number of books in the end = 1 u + 4
Total cost of books at first
= 10 x 1 u
= 10 u
Total cost of books in the end
= 16 x (1 u + 4)
= 16 u + 64
16 u + 64 = 10 u + 100
16 u - 10 u = 100 - 64
6 u = 36
1 u = 36 ÷ 6 = 6
Number of books at first
= 1 u
= 6
Answer(s): 6