The average cost of some books is $36.
If 3 of the books are now each $10 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 |
36 |
39 |
Total value |
36 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
= 36 x 1 u
= 36 u
Total cost of books in the end
= 39 x 1 u
= 39 u
Increase in the cost when 3 books are now each $10 more
= 3 x 10
= $30
Total cost of books after 3 books are now each $10 more
= 36 u + 30
39 u = 36 u + 30
39 u - 36 u = 30
3 u = 30
1 u = 30 ÷ 3 = 10
Number of books in the end
= 1 u
= 10
Answer(s): 10