How do you find the tax on a purchase and calculate the final bill? give an example, then write and solve a number sentence to illustrate your strategy.

If you are buying something that cost $15 and sale tax is 6%, you can calculate final bill trough proportions:
$15:100%=$x:106%
100x=106*15, x=(106*15)/100, x=$15.9  is final bill.

You can also use easier way: $15+0.06*$15=$15.9
Explanation for 2nd way: Cost is $15, on that you need to add 6% of $15: (6%/100%)*$15 and that is 0.06*15.

The answer is: y would equal 3 because if you simplify the equation then it would be y= 3x+9 and if you divide 3x and 9 by 3 it would equal 3. y= 3