The average cost of some books is $55.
If 7 of the books are now each $13 more and
2 of the books are now each $5 less,
the average cost of the books becomes $58.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
55 |
58 |
Total value |
55 u |
58 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 55 x 1 u
= 55 u
Increase in the cost after 7 books are now each $13 more and 2 books are now each $5 less
= 7 x 13 - 2 x 5
= 91 - 10
= $81
Total cost of books after 7 books are now each $13 more and 2 books are now each $5 less
= 55 u + 81
Total cost of books in the end
= 58 x 1 u
= 58 u
58 u = 55 u + 81
58 u - 55 u = 81
3 u = 81
1 u = 81 ÷ 3 = 27
Number of books in the end
= 1 u
= 27
Answer(s): 27