The average cost of some books is $70.
When a book that costs $94 is added,
the average cost of the books becomes $72.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 1 |
Value |
70 |
72 |
Total value |
70 u |
72 u + 72 |
Number of books at first = 1 u
Number of books in the end = 1 u + 1
Total cost of the books at first
= 70 x 1 u
= 70 u
Total cost of books in the end
= 70 u + 94
Total cost of books in the end
= 72 x (1 u + 1)
= 72 u + 72
70 u + 94 = 72 u + 72
72 u - 70 u = 94 - 72
2 u = 22
1 u = 22 ÷ 2 = 11
Number of books in the end
= 1 u + 1
= 11 + 1
= 12
Answer(s): 12