You buy six notebooks and a backpack. the cost of the backpack is$ 17.90. you pay 6% in sales tax. your total bill is $30.74 how much does each notebook cost?

Set this up as an equation to start out. We'll call the a notebook n. 
First you need to subtract the sales tax from the total price...30.74 - (30.74*.06) = 17.9 + 6n 
Simplify
28.92 = 17.9 + 6n
Subtract 17.9 from both sides. 
11.02 = 6n
Divide both sides by 6
1.8367 = n (so this is the cost of 1 notebook.)

$1.85 per notebook
Check:
($1.85)(6)=$11.10
$11.10+$17.90=$29.00
($29.00)(.06)=$1.74
$29.00+$1.74=$30.74

1. 3/6 or 1/22. 1/123. 1/84. 1/105. 2/12 or 1/6