The average cost of some books is $42.
If 5 of the books are now each $15 more,
the average cost of the books becomes $47.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
42 |
47 |
Total value |
42 u |
47 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 42 x 1 u
= 42 u
Total cost of books in the end
= 47 x 1 u
= 47 u
Increase in the cost when 5 books are now each $15 more
= 5 x 15
= $75
Total cost of books after 5 books are now each $15 more
= 42 u + 75
47 u = 42 u + 75
47 u - 42 u = 75
5 u = 75
1 u = 75 ÷ 5 = 15
Number of books in the end
= 1 u
= 15
Answer(s): 15