The average cost of some books is $50.
If 5 of the books are now each $15 more,
the average cost of the books becomes $55.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
50 |
55 |
Total value |
50 u |
55 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 50 x 1 u
= 50 u
Total cost of books in the end
= 55 x 1 u
= 55 u
Increase in the cost when 5 books are now each $15 more
= 5 x 15
= $75
Total cost of books after 5 books are now each $15 more
= 50 u + 75
55 u = 50 u + 75
55 u - 50 u = 75
5 u = 75
1 u = 75 ÷ 5 = 15
Number of books in the end
= 1 u
= 15
Answer(s): 15