The average cost of some books is $10.
After 2 new books are added,
it adds another $56 to the total cost.
As a result, the average cost of all the books increases to $14.
How many books are there at first?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 2 |
Value |
10 |
14 |
Total value |
10 u |
14 u + 28 |
Number of books at first = 1 u
Number of books in the end = 1 u + 2
Total cost of books at first
= 10 x 1 u
= 10 u
Total cost of books in the end
= 14 x (1 u + 2)
= 14 u + 28
14 u + 28 = 10 u + 56
14 u - 10 u = 56 - 28
4 u = 28
1 u = 28 ÷ 4 = 7
Number of books at first
= 1 u
= 7
Answer(s): 7