The average cost of some books is $40.
When a book that costs $84 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 + 84
Total cost of books in the end
= 42 x (1 u + 1)
= 42 u + 42
40 u + 84 = 42 u + 42
42 u - 40 u = 84 - 42
2 u = 42
1 u = 42 ÷ 2 = 21
Number of books in the end
= 1 u + 1
= 21 + 1
= 22
Answer(s): 22