The average cost of some books is $10.
After 2 new books are added,
it adds another $100 to the total cost.
As a result, the average cost of all the books increases to $18.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
10 |
18 |
Total value |
10 u |
18 u + 36 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 10 x 1 u
= 10 u
Total cost of books in the end
= 18 x (1 u + 2)
= 18 u + 36
18 u + 36 = 10 u + 100
18 u - 10 u = 100 - 36
8 u = 64
1 u = 64 ÷ 8 = 8
Number of books at first
= 1 u
= 8
Answer(s): 8