To add $3.9 + \frac{1}{3}$:

1. Convert $3.9$ to a fraction:
   $3.9 = \frac{39}{10}$.

2. Add the fractions:
   $\frac{39}{10} + \frac{1}{3}$.

3. Find the least common denominator (LCD) of 10 and 3, which is 30.

4. Convert both fractions to have a common denominator:
   $\frac{39}{10} = \frac{39 \times 3}{10 \times 3} = \frac{117}{30}$,
   $\frac{1}{3} = \frac{1 \times 10}{3 \times 10} = \frac{10}{30}$.

5. Add the fractions:
   $\frac{117}{30} + \frac{10}{30} = \frac{127}{30}$.

6. Convert $\frac{127}{30}$ to a decimal:
   $\frac{127}{30} \approx 4.2333\ldots$.

So, $3.9 + \frac{1}{3} \approx 4.2333\ldots$ or $\frac{127}{30}$ in fraction form.

Would you like further details or have any questions?

Here are some related questions:

1. How do you convert a decimal to a fraction?
2. How do you add fractions with different denominators?
3. What is the process for finding the least common denominator?
4. How do you simplify fractions?
5. How do you convert a repeating decimal to a fraction?

Tip: When adding fractions, always find a common denominator to combine them efficiently.