The average cost of some books is $60.
If 9 of the books are now each $12 more and
3 of the books are now each $6 less,
the average cost of the books becomes $63.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
60 |
63 |
Total value |
60 u |
63 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 60 x 1 u
= 60 u
Increase in the cost after 9 books are now each $12 more and 3 books are now each $6 less
= 9 x 12 - 3 x 6
= 108 - 18
= $90
Total cost of books after 9 books are now each $12 more and 3 books are now each $6 less
= 60 u + 90
Total cost of books in the end
= 63 x 1 u
= 63 u
63 u = 60 u + 90
63 u - 60 u = 90
3 u = 90
1 u = 90 ÷ 3 = 30
Number of books in the end
= 1 u
= 30
Answer(s): 30