To find the percentage of people who had a mild allergic reaction to the new shampoo, you would set up the percentage proportion as you indicated. The proportion can be written as:
\[ \frac{54}{9000} = \frac{p}{100} \]
where \( p \) represents the percentage of people who had a mild allergic reaction.
To solve for \( p \), cross-multiply:
\[ 54 \times 100 = p \times 9000 \]
This simplifies to:
\[ 5400 = p \times 9000 \]
Next, divide both sides by 9000 to isolate \( p \):
\[ p = \frac{5400}{9000} \]
Calculating that gives:
\[ p = 0.6 \]
To convert this to a percentage, multiply by 100:
\[ p = 0.6 \times 100 = 60 \]
Therefore, \( 60% \) of the people had a mild allergic reaction.
So the answer to the original question is:
\[ 60% \]
The complete proportion you set up is:
\[ \frac{54}{9000} = \frac{60}{100} \]
This indicates that 60 out of every 100 people (or 60%) had a mild allergic reaction.