The average cost of some books is $30.
If 6 of the books are now each $13 more and
2 of the books are now each $4 less,
the average cost of the books becomes $32.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
30 |
32 |
Total value |
30 u |
32 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 6 books are now each $13 more and 2 books are now each $4 less
= 6 x 13 - 2 x 4
= 78 - 8
= $70
Total cost of books after 6 books are now each $13 more and 2 books are now each $4 less
= 30 u + 70
Total cost of books in the end
= 32 x 1 u
= 32 u
32 u = 30 u + 70
32 u - 30 u = 70
2 u = 70
1 u = 70 ÷ 2 = 35
Number of books in the end
= 1 u
= 35
Answer(s): 35