10 yards. The amount of yards is 1 third the amount of feet so you just divide 30 by 3 to get the amount of yards
Let's assume the number be x.
So, sum of a number and 10 can be written as x+10.
Now the given statement is "One half of the sum of a number and 10 ".
So, one half of x+10 can be written as 
First calculate after tax tire price
$390.60 / 4 = $97.65 after tax price per tire
take the $97.65 divided by 90
97.65 / 90 = 1.085
1.085 is the rate of tax he paid per tire
Is that the whole question because you cant really solve this without all the details.<span />