An item costs $420 before tax, and the sales tax is$ 16.80 whats the sale tax rate

We need to set up an equation to solve this problem.  The sales tax is a percentage of the total, so that's what we are looking for.

Let's call S our sales tax

So, that's what we are looking for, and we are given the actual sales tax

So now we have a decimal.  In order to convert a decimal into a percentage, we must multiply by 100 (since percentages are out of 100)

The sales tax rate is 3.8%

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional