The average cost of some books is $60.
If 5 of the books are now each $15 more and
2 of the books are now each $6 less,
the average cost of the books becomes $63.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
60 |
63 |
Total value |
60 u |
63 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 60 x 1 u
= 60 u
Increase in the cost after 5 books are now each $15 more and 2 books are now each $6 less
= 5 x 15 - 2 x 6
= 75 - 12
= $63
Total cost of books after 5 books are now each $15 more and 2 books are now each $6 less
= 60 u + 63
Total cost of books in the end
= 63 x 1 u
= 63 u
63 u = 60 u + 63
63 u - 60 u = 63
3 u = 63
1 u = 63 ÷ 3 = 21
Number of books in the end
= 1 u
= 21
Answer(s): 21