The average cost of some books is $39.
If 5 of the books are now each $12 more,
the average cost of the books becomes $41.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
39 |
41 |
Total value |
39 u |
41 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 39 x 1 u
= 39 u
Total cost of books in the end
= 41 x 1 u
= 41 u
Increase in the cost when 5 books are now each $12 more
= 5 x 12
= $60
Total cost of books after 5 books are now each $12 more
= 39 u + 60
41 u = 39 u + 60
41 u - 39 u = 60
2 u = 60
1 u = 60 ÷ 2 = 30
Number of books in the end
= 1 u
= 30
Answer(s): 30