The average cost of some books is $40.
If 7 of the books are now each $8 more and
3 of the books are now each $8 less,
the average cost of the books becomes $42.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
40 |
42 |
Total value |
40 u |
42 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 40 x 1 u
= 40 u
Increase in the cost after 7 books are now each $8 more and 3 books are now each $8 less
= 7 x 8 - 3 x 8
= 56 - 24
= $32
Total cost of books after 7 books are now each $8 more and 3 books are now each $8 less
= 40 u + 32
Total cost of books in the end
= 42 x 1 u
= 42 u
42 u = 40 u + 32
42 u - 40 u = 32
2 u = 32
1 u = 32 ÷ 2 = 16
Number of books in the end
= 1 u
= 16
Answer(s): 16