The average cost of some books is $30.
If 7 of the books are now each $14 more and
2 of the books are now each $7 less,
the average cost of the books becomes $33.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
30 |
33 |
Total value |
30 u |
33 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 30 x 1 u
= 30 u
Increase in the cost after 7 books are now each $14 more and 2 books are now each $7 less
= 7 x 14 - 2 x 7
= 98 - 14
= $84
Total cost of books after 7 books are now each $14 more and 2 books are now each $7 less
= 30 u + 84
Total cost of books in the end
= 33 x 1 u
= 33 u
33 u = 30 u + 84
33 u - 30 u = 84
3 u = 84
1 u = 84 ÷ 3 = 28
Number of books in the end
= 1 u
= 28
Answer(s): 28