Let $R$ be the rectangle in the Cartesian plane with vertices at $(0,0)$, $(2,0)$, $(2,1)$, and $(0,1)$. $R$ can be divided into two unit squares, as shown. [asy]size(120); defaultpen(linewidth(0.7)); 
draw(origin--(2,0)--(2,1)--(0,1)--cycle^^(1,0)--(1,1));[/asy] Pro selects a point $P$ at random in the interior of $R$. Find the probability that the line through $P$ with slope $\frac{1}{2}$ will pass through both unit squares.
1. **Identify the problem and the region of interest:**
   - We are given a rectangle \( R \) with vertices at \((0,0)\), \((2,0)\), \((2,1)\), and \((0,1)\).
   - The rectangle \( R \) is divided into two unit squares: the left square with vertices \((0,0)\), \((1,0)\), \((1,1)\), and \((0,1)\), and the right square with vertices \((1,0)\), \((2,0)\), \((2,1)\), and \((1,1)\).

2. **Determine the condition for the line to pass through both squares:**
   - A point \( P \) is selected randomly in the interior of \( R \).
   - We need to find the probability that the line through \( P \) with slope \(\frac{1}{2}\) will pass through both unit squares.

3. **Analyze the line passing through \( P \):**
   - Let \( P = (x_1, y_1) \).
   - The equation of the line through \( P \) with slope \(\frac{1}{2}\) is \( y - y_1 = \frac{1}{2}(x - x_1) \), which simplifies to \( y = \frac{1}{2}x + b \) where \( b = y_1 - \frac{1}{2}x_1 \).

4. **Condition for the line to pass through both squares:**
   - The line must intersect the vertical line \( x = 1 \) within the interval \( 0 \leq y \leq 1 \).
   - Substitute \( x = 1 \) into the line equation: \( y = \frac{1}{2}(1) + b = \frac{1}{2} + b \).
   - For the line to intersect within the interval \( 0 \leq y \leq 1 \), we need \( 0 \leq \frac{1}{2} + b \leq 1 \).

5. **Solve for \( b \):**
   - \( 0 \leq \frac{1}{2} + b \leq 1 \)
   - Subtract \(\frac{1}{2}\) from all parts: \( -\frac{1}{2} \leq b \leq \frac{1}{2} \).

6. **Determine the range of \( b \):**
   - The total possible range for \( b \) is from \(-\frac{1}{2}\) to \(\frac{1}{2}\), which is a length of 1.
   - The range of \( b \) that satisfies the condition is from \( 0 \) to \(\frac{1}{2}\), which is a length of \(\frac{1}{2}\).

7. **Calculate the probability:**
   - The probability that \( b \) falls within the desired range is the length of the desired range divided by the total range: \(\frac{\frac{1}{2}}{1} = \frac{1}{2}\).

8. **Combine probabilities for both squares:**
   - If \( P \) is in the right square, the line will always pass through both squares.
   - If \( P \) is in the left square, the probability is \(\frac{1}{2}\).
   - The total probability is the sum of the probabilities for each case, weighted by the probability of \( P \) being in each square:
     \[
     \frac{1}{2} \cdot 1 + \frac{1}{2} \cdot \frac{1}{2} = \frac{1}{2} + \frac{1}{4} = \frac{3}{4}.
     \]

The final answer is \(\boxed{\frac{3}{4}}\).