The average cost of some books is $50.
If 4 of the books are now each $13 more and
2 of the books are now each $5 less,
the average cost of the books becomes $52.
How many books are there altogether?

	Case 1	Case 2
Number	1 u	1 u
Value	50	52
Total value	50 u	52 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

Increase in the cost after 4 books are now each $13 more and 2 books are now each $5 less
= 4 x 13 - 2 x 5
= 52 - 10
= $42

Total cost of books after 4 books are now each $13 more and 2 books are now each $5 less
= 50 u + 42

Total cost of books in the end
= 52 x 1 u
= 52 u

52 u = 50 u + 42
52 u - 50 u = 42
2 u = 42

1 u = 42 ÷ 2 = 21

Number of books in the end
= 1 u
= 21

Answer(s): 21