Question 1176483
Bob bought some books at an average cost of $23.85 each.
 He mistakenly bought a book for $48.90 instead of another book that cost $12.90.
 The average cost of the books that Bob bought should have been $19.35.
 How many books did he buy.
:
let n = number of books bought
let a = total amt spent on the books not given here
:
Two equations
{{{((a + 48.90))/n}}} = 23.85
and
{{{((a + 12.90))/n}}} = 19.35
:
multiply by n, we can use elimination here
a + 48.90 = 23.85n
a + 12.90 = 19.35n
---------------------Subtraction eliminates a
0 + 36 = 4.50n
n = 36/4.5
n = 8 books