To find the percentage of tax, you can use the following formula:
\[ \text{Tax Percentage} = \left( \frac{\text{Tax Amount}}{\text{Cost of the Sandwich}} \right) \times 100 \]
In this case, the tax amount is $0.27 and the cost of the sandwich is $4.50.
Now, substituting these values into the formula:
\[ \text{Tax Percentage} = \left( \frac{0.27}{4.50} \right) \times 100 \]
Calculating the fraction:
\[ \frac{0.27}{4.50} = 0.06 \]
Now, multiply by 100 to convert it to a percentage:
\[ 0.06 \times 100 = 6 \]
Therefore, the percentage of tax is 6%.