The average cost of some books is $30.
After 2 new books are added,
it adds another $96 to the total cost.
As a result, the average cost of all the books increases to $34.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
30 |
34 |
Total value |
30 u |
34 u + 68 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 34 x (1 u + 2)
= 34 u + 68
34 u + 68 = 30 u + 96
34 u - 30 u = 96 - 68
4 u = 28
1 u = 28 ÷ 4 = 7
Number of books at first
= 1 u
= 7
Answer(s): 7