The average cost of some books is $42.
When another 6 books are added,
the total cost increases by $210.
The average cost becomes $40.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 6 |
Value |
42 |
40 |
Total value |
42 u |
40 u + 240 |
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 42
= 42 u
Total cost of the books in the end
= 40 x (1 u + 6)
= 40 u + 240
42 u + 210 = 40 u + 240
42 u - 40 u = 240 - 210
2 u = 30
1 u = 30 ÷ 2 = 15
Number of books in the end
= 1 u + 6
= 15 + 6
= 21
Answer(s): 21