The average cost of some books is $34.
If 3 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 3 books are now each $15 more
= 3 x 15
= $45
Total cost of books after 3 books are now each $15 more
= 34 u + 45
39 u = 34 u + 45
39 u - 34 u = 45
5 u = 45
1 u = 45 ÷ 5 = 9
Number of books in the end
= 1 u
= 9
Answer(s): 9