Complete the Decimal Expression: 3.1 = ?

To solve this problem, we need to convert the decimal $3.1$ into a mixed number:

• Step 1: Recognize the decimal number $3.1$ consists of a whole number part $3$ and a decimal part $0.1$.
• Step 2: Convert the decimal part $0.1$ to a fraction. The decimal $0.1$ is equivalent to $\frac{1}{10}$.
• Step 3: Combine the whole number part with the fractional part to form the mixed number, which gives us $3\frac{1}{10}$.

Therefore, the decimal $3.1$ is equivalent to the mixed number $3\frac{1}{10}$.

The correct choice is option 1: $3\frac{1}{10}$.