The average cost of some books is $39.
If 3 of the books are now each $12 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 3 books are now each $12 more
= 3 x 12
= $36
Total cost of books after 3 books are now each $12 more
= 39 u + 36
42 u = 39 u + 36
42 u - 39 u = 36
3 u = 36
1 u = 36 ÷ 3 = 12
Number of books in the end
= 1 u
= 12
Answer(s): 12