The average cost of some books is $36.
When another 7 books are added,
the total cost increases by $120.
The average cost becomes $30.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 7 |
Value |
36 |
30 |
Total value |
36 u |
30 u + 210 |
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 36
= 36 u
Total cost of the books in the end
= 30 x (1 u + 7)
= 30 u + 210
36 u + 120 = 30 u + 210
36 u - 30 u = 210 - 120
6 u = 90
1 u = 90 ÷ 6 = 15
Number of books in the end
= 1 u + 7
= 15 + 7
= 22
Answer(s): 22