The average cost of some books is $70.
When another 8 books are added,
the total cost increases by $100.
The average cost becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 8 |
Value |
70 |
50 |
Total value |
70 u |
50 u + 400 |
Number of books at first = 1 u
Number of books in the end = 1 u + 8
Total cost of books at first
= 1 u x 70
= 70 u
Total cost of the books in the end
= 50 x (1 u + 8)
= 50 u + 400
70 u + 100 = 50 u + 400
70 u - 50 u = 400 - 100
20 u = 300
1 u = 300 ÷ 20 = 15
Number of books in the end
= 1 u + 8
= 15 + 8
= 23
Answer(s): 23