The average cost of some books is $42.
If 3 of the books are now each $13 more,
the average cost of the books becomes $45.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
42 |
45 |
Total value |
42 u |
45 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 42 x 1 u
= 42 u
Total cost of books in the end
= 45 x 1 u
= 45 u
Increase in the cost when 3 books are now each $13 more
= 3 x 13
= $39
Total cost of books after 3 books are now each $13 more
= 42 u + 39
45 u = 42 u + 39
45 u - 42 u = 39
3 u = 39
1 u = 39 ÷ 3 = 13
Number of books in the end
= 1 u
= 13
Answer(s): 13