The average cost of some books is $39.
If 2 of the books are now each $15 more,
the average cost of the books becomes $42.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
39 |
42 |
Total value |
39 u |
42 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 39 x 1 u
= 39 u
Total cost of books in the end
= 42 x 1 u
= 42 u
Increase in the cost when 2 books are now each $15 more
= 2 x 15
= $30
Total cost of books after 2 books are now each $15 more
= 39 u + 30
42 u = 39 u + 30
42 u - 39 u = 30
3 u = 30
1 u = 30 ÷ 3 = 10
Number of books in the end
= 1 u
= 10
Answer(s): 10