The average cost of some books is $70.
When another 7 books are added,
the total cost increases by $110.
The average cost becomes $50.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
70 |
50 |
Total value |
70 u |
50 u + 350 |
Number of books at first = 1 u
Number of books in the end = 1 u + 7
Total cost of books at first
= 1 u x 70
= 70 u
Total cost of the books in the end
= 50 x (1 u + 7)
= 50 u + 350
70 u + 110 = 50 u + 350
70 u - 50 u = 350 - 110
20 u = 240
1 u = 240 ÷ 20 = 12
Number of books in the end
= 1 u + 7
= 12 + 7
= 19
Answer(s): 19