The average cost of some books is $35.
If 4 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 |
35 |
39 |
Total value |
35 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
= 35 x 1 u
= 35 u
Total cost of books in the end
= 39 x 1 u
= 39 u
Increase in the cost when 4 books are now each $10 more
= 4 x 10
= $40
Total cost of books after 4 books are now each $10 more
= 35 u + 40
39 u = 35 u + 40
39 u - 35 u = 40
4 u = 40
1 u = 40 ÷ 4 = 10
Number of books in the end
= 1 u
= 10
Answer(s): 10