The average cost of some books is $20.
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 $25.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
20 |
25 |
Total value |
20 u |
25 u + 50 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 20 x 1 u
= 20 u
Total cost of books in the end
= 25 x (1 u + 2)
= 25 u + 50
25 u + 50 = 20 u + 100
25 u - 20 u = 100 - 50
5 u = 50
1 u = 50 ÷ 5 = 10
Number of books at first
= 1 u
= 10
Answer(s): 10