The average cost of some books is $49.
If 4 of the books are now each $10 more,
the average cost of the books becomes $51.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
49 |
51 |
Total value |
49 u |
51 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 49 x 1 u
= 49 u
Total cost of books in the end
= 51 x 1 u
= 51 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
= 49 u + 40
51 u = 49 u + 40
51 u - 49 u = 40
2 u = 40
1 u = 40 ÷ 2 = 20
Number of books in the end
= 1 u
= 20
Answer(s): 20