The average cost of some books is $36.
When another 6 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 + 6 |
Value |
36 |
30 |
Total value |
36 u |
30 u + 180 |
Number of books at first = 1 u
Number of books in the end = 1 u + 6
Total cost of books at first
= 1 u x 36
= 36 u
Total cost of the books in the end
= 30 x (1 u + 6)
= 30 u + 180
36 u + 120 = 30 u + 180
36 u - 30 u = 180 - 120
6 u = 60
1 u = 60 ÷ 6 = 10
Number of books in the end
= 1 u + 6
= 10 + 6
= 16
Answer(s): 16