The average cost of some books is $35.
If 9 of the books are now each $12 more and
4 of the books are now each $6 less,
the average cost of the books becomes $37.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
35 |
37 |
Total value |
35 u |
37 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 35 x 1 u
= 35 u
Increase in the cost after 9 books are now each $12 more and 4 books are now each $6 less
= 9 x 12 - 4 x 6
= 108 - 24
= $84
Total cost of books after 9 books are now each $12 more and 4 books are now each $6 less
= 35 u + 84
Total cost of books in the end
= 37 x 1 u
= 37 u
37 u = 35 u + 84
37 u - 35 u = 84
2 u = 84
1 u = 84 ÷ 2 = 42
Number of books in the end
= 1 u
= 42
Answer(s): 42