The correct answer is: $0.3937\text{ in}$
Explanation
Convert using $1\text{ in}=25.4\text{ mm}$.
Steps:
- Relationship: $$1\text{ in}=25.4\text{ mm}$$
- Convert: $$10\text{ mm}=\frac{10}{25.4}\text{ in}=0.3937007874\text{ in}$$
- Rounded for typical use: $$\approx0.394\text{ in}$$
Therefore, 10 mm is about 0.394 inches — on a standard inch ruler it falls just past the 3/8″ mark (3/8″ = $0.375\text{ in}=9.525\text{ mm}$).