The average cost of some books is $40.
When a book that costs $78 is added,
the average cost of the books becomes $42.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
40 |
42 |
Total value |
40 u |
42 u + 42 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 40 x 1 u
= 40 u
Total cost of books in the end
= 40 u + 78
Total cost of books in the end
= 42 x (1 u + 1)
= 42 u + 42
40 u + 78 = 42 u + 42
42 u - 40 u = 78 - 42
2 u = 36
1 u = 36 ÷ 2 = 18
Number of books in the end
= 1 u + 1
= 18 + 1
= 19
Answer(s): 19