The average cost of some books is $31.
If 3 of the books are now each $12 more,
the average cost of the books becomes $35.
How many books are there altogether?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u |
Value |
31 |
35 |
Total value |
31 u |
35 u |
Number of books at first = 1 u
Number of books in the end = 1 u
Total cost of the books at first
= 31 x 1 u
= 31 u
Total cost of books in the end
= 35 x 1 u
= 35 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
= 31 u + 36
35 u = 31 u + 36
35 u - 31 u = 36
4 u = 36
1 u = 36 ÷ 4 = 9
Number of books in the end
= 1 u
= 9
Answer(s): 9