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 $20.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
10 |
20 |
Total value |
10 u |
20 u + 40 |
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
= 20 x (1 u + 2)
= 20 u + 40
20 u + 40 = 10 u + 100
20 u - 10 u = 100 - 40
10 u = 60
1 u = 60 ÷ 10 = 6
Number of books at first
= 1 u
= 6
Answer(s): 6