To find the thickness of a fingernail, we need to add the thickness of the hair to the thickness difference provided.
The diameter of a human hair is: \[ 4 \times 10^{-3} \text{ inches} \] The thickness difference is: \[ 0.011 \text{ inches} \]
Now, we add these two values together: \[ \text{Thickness of fingernail} = 4 \times 10^{-3} + 0.011 \]
First, convert \( 4 \times 10^{-3} \) to decimal form: \[ 4 \times 10^{-3} = 0.004 \]
Now, add this to the thickness difference: \[ 0.004 + 0.011 = 0.015 \]
Thus, the thickness of a fingernail, rounded to three decimal places, is: \[ \boxed{0.015} \text{ inches} \]