The average cost of some books is $80.
When another 5 books are added,
the total cost increases by $150.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 5 |
Value |
80 |
70 |
Total value |
80 u |
70 u + 350 |
Number of books at first = 1 u
Number of books in the end = 1 u + 5
Total cost of books at first
= 1 u x 80
= 80 u
Total cost of the books in the end
= 70 x (1 u + 5)
= 70 u + 350
80 u + 150 = 70 u + 350
80 u - 70 u = 350 - 150
10 u = 200
1 u = 200 ÷ 10 = 20
Number of books in the end
= 1 u + 5
= 20 + 5
= 25
Answer(s): 25