The average cost of some books is $45.
If 2 of the books are now each $10 more,
the average cost of the books becomes $47.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
45 |
47 |
Total value |
45 u |
47 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 45 x 1 u
= 45 u
Total cost of books in the end
= 47 x 1 u
= 47 u
Increase in the cost when 2 books are now each $10 more
= 2 x 10
= $20
Total cost of books after 2 books are now each $10 more
= 45 u + 20
47 u = 45 u + 20
47 u - 45 u = 20
2 u = 20
1 u = 20 ÷ 2 = 10
Number of books in the end
= 1 u
= 10
Answer(s): 10