The average cost of some books is $20.
After 2 new books are added,
it adds another $84 to the total cost.
As a result, the average cost of all the books increases to $24.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
20 |
24 |
Total value |
20 u |
24 u + 48 |
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
= 24 x (1 u + 2)
= 24 u + 48
24 u + 48 = 20 u + 84
24 u - 20 u = 84 - 48
4 u = 36
1 u = 36 ÷ 4 = 9
Number of books at first
= 1 u
= 9
Answer(s): 9