The average cost of some books is $74.
When another 3 books are added,
the total cost increases by $170.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 3 |
Value |
74 |
70 |
Total value |
74 u |
70 u + 210 |
Number of books at first = 1 u
Number of books in the end = 1 u + 3
Total cost of books at first
= 1 u x 74
= 74 u
Total cost of the books in the end
= 70 x (1 u + 3)
= 70 u + 210
74 u + 170 = 70 u + 210
74 u - 70 u = 210 - 170
4 u = 40
1 u = 40 ÷ 4 = 10
Number of books in the end
= 1 u + 3
= 10 + 3
= 13
Answer(s): 13