The average cost of some books is $30.
When a book that costs $78 is added,
the average cost of the books becomes $34.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
30 |
34 |
Total value |
30 u |
34 u + 34 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 30 x 1 u
= 30 u
Total cost of books in the end
= 30 u + 78
Total cost of books in the end
= 34 x (1 u + 1)
= 34 u + 34
30 u + 78 = 34 u + 34
34 u - 30 u = 78 - 34
4 u = 44
1 u = 44 ÷ 4 = 11
Number of books in the end
= 1 u + 1
= 11 + 1
= 12
Answer(s): 12