The average cost of some books is $40.
If 3 of the books are now each $10 more,
the average cost of the books becomes $43.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
40 |
43 |
Total value |
40 u |
43 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 40 x 1 u
= 40 u
Total cost of books in the end
= 43 x 1 u
= 43 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
= 40 u + 30
43 u = 40 u + 30
43 u - 40 u = 30
3 u = 30
1 u = 30 ÷ 3 = 10
Number of books in the end
= 1 u
= 10
Answer(s): 10