The average cost of some books is $45.
If 4 of the books are now each $13 more and
2 of the books are now each $7 less,
the average cost of the books becomes $47.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
45 |
47 |
Total value |
45 u |
47 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 45 x 1 u
= 45 u
Increase in the cost after 4 books are now each $13 more and 2 books are now each $7 less
= 4 x 13 - 2 x 7
= 52 - 14
= $38
Total cost of books after 4 books are now each $13 more and 2 books are now each $7 less
= 45 u + 38
Total cost of books in the end
= 47 x 1 u
= 47 u
47 u = 45 u + 38
47 u - 45 u = 38
2 u = 38
1 u = 38 ÷ 2 = 19
Number of books in the end
= 1 u
= 19
Answer(s): 19