Problem 194

### Solution:

#### Part 1:

Given:
- Total number of balls in the bag = $10$
- Number of yellow balls = $6$

The probability of drawing a yellow ball is calculated as the ratio of the number of favorable outcomes (drawing a yellow ball) to the total number of possible outcomes (drawing any ball).

Thus, the probability of drawing a yellow ball is:
$$P(\text{Yellow}) = \frac{\text{Number of yellow balls}}{\text{Total number of balls}} = \frac{6}{10} = \frac{3}{5}$$

So, the probability of drawing a yellow ball is $\boxed{\frac{3}{5}}$.

#### Part 2:

Let $x$ represent the number of additional red balls needed.

The total number of balls after adding $x$ red balls becomes $10 + x$. The number of red balls becomes $4 + x$.

Given that the probability of drawing a red ball should be $\frac{2}{3}$, we set up the equation based on the definition of probability:
$$\frac{4 + x}{10 + x} = \frac{2}{3}$$

Solving for $x$:
$$\begin{align*} 3(4 + x) &= 2(10 + x) \\ 12 + 3x &= 20 + 2x \\ 3x - 2x &= 20 - 12 \\ x &= 8 \end{align*}$$

After solving, we find that $x = 8$ satisfies the equation, meaning $8$ more red balls need to be added to the bag to make the probability of drawing a red ball $\frac{2}{3}$.

Therefore, the number of additional red balls needed is $\boxed{8}$.