The average cost of some books is $35.
If 9 of the books are now each $15 more and
6 of the books are now each $7 less,
the average cost of the books becomes $38.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
35 |
38 |
Total value |
35 u |
38 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 $15 more and 6 books are now each $7 less
= 9 x 15 - 6 x 7
= 135 - 42
= $93
Total cost of books after 9 books are now each $15 more and 6 books are now each $7 less
= 35 u + 93
Total cost of books in the end
= 38 x 1 u
= 38 u
38 u = 35 u + 93
38 u - 35 u = 93
3 u = 93
1 u = 93 ÷ 3 = 31
Number of books in the end
= 1 u
= 31
Answer(s): 31