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

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

2. Now, add it to $\frac{1}{3}$: $\frac{39}{10} + \frac{1}{3}$

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

4. Rewrite the fractions with the denominator of 30: $\frac{39}{10} = \frac{117}{30} \quad \text{and} \quad \frac{1}{3} = \frac{10}{30}$

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

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

So, $3.9 + \frac{1}{3} \approx 4.2333$.

Would you like further details or have any questions?

Here are 5 related questions:

1. How do you convert a repeating decimal into a fraction?
2. How do you find the least common denominator for multiple fractions?
3. What is the general method to add fractions with different denominators?
4. How do you convert improper fractions back to decimals?
5. How do you simplify fractions?

Tip: Always check if fractions can be simplified before adding them for easier calculation.