The average cost of some books is $34.
If 5 of the books are now each $15 more,
the average cost of the books becomes $39.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
34 |
39 |
Total value |
34 u |
39 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 34 x 1 u
= 34 u
Total cost of books in the end
= 39 x 1 u
= 39 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
= 34 u + 75
39 u = 34 u + 75
39 u - 34 u = 75
5 u = 75
1 u = 75 ÷ 5 = 15
Number of books in the end
= 1 u
= 15
Answer(s): 15