The average cost of some books is $45.
If 7 of the books are now each $15 more and
3 of the books are now each $5 less,
the average cost of the books becomes $48.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
45 |
48 |
Total value |
45 u |
48 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 7 books are now each $15 more and 3 books are now each $5 less
= 7 x 15 - 3 x 5
= 105 - 15
= $90
Total cost of books after 7 books are now each $15 more and 3 books are now each $5 less
= 45 u + 90
Total cost of books in the end
= 48 x 1 u
= 48 u
48 u = 45 u + 90
48 u - 45 u = 90
3 u = 90
1 u = 90 ÷ 3 = 30
Number of books in the end
= 1 u
= 30
Answer(s): 30