The average cost of some books is $40.
If 9 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 $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 9 books are now each $14 more and 2 books are now each $7 less
= 9 x 14 - 2 x 7
= 126 - 14
= $112
Total cost of books after 9 books are now each $14 more and 2 books are now each $7 less
= 40 u + 112
Total cost of books in the end
= 42 x 1 u
= 42 u
42 u = 40 u + 112
42 u - 40 u = 112
2 u = 112
1 u = 112 ÷ 2 = 56
Number of books in the end
= 1 u
= 56
Answer(s): 56