 A right cylinder is given with a height of $20$ and a circular base of radius $5$. A vertical planar cut is made into this base of radius $5$. A vertical planar cut, perpendicular to the base, is made into this cylinder, splitting the cylinder into two pieces. Suppose the area the cut leaves behind on one of the pieces is $100\sqrt2$. Then the volume of the larger piece can be written as $a + b\pi$, where $a, b$ are positive integers. Find $a + b$.
1. **Identify the given parameters and the problem setup:**
   - Height of the cylinder, \( h = 20 \)
   - Radius of the base, \( r = 5 \)
   - Area of the cut left behind on one of the pieces, \( 100\sqrt{2} \)

2. **Determine the length of the chord \( AB \) formed by the cut:**
   - The area of the cut is given as \( 100\sqrt{2} \).
   - Since the height of the cylinder is \( 20 \), the length of the chord \( AB \) can be calculated as:
     \[
     AB = \frac{100\sqrt{2}}{20} = 5\sqrt{2}
     \]

3. **Use the properties of the circle to find the angle \( \angle AOB \):**
   - Given \( AB = 5\sqrt{2} \) and \( OA = OB = 5 \), we can use the cosine rule in \(\triangle OAB\):
     \[
     AB^2 = OA^2 + OB^2 - 2 \cdot OA \cdot OB \cdot \cos(\angle AOB)
     \]
     \[
     (5\sqrt{2})^2 = 5^2 + 5^2 - 2 \cdot 5 \cdot 5 \cdot \cos(\angle AOB)
     \]
     \[
     50 = 25 + 25 - 50 \cos(\angle AOB)
     \]
     \[
     50 = 50 - 50 \cos(\angle AOB)
     \]
     \[
     \cos(\angle AOB) = 0 \implies \angle AOB = 90^\circ
     \]

4. **Calculate the area of the larger segment of the base circle:**
   - The angle \( \angle AOB = 90^\circ \) divides the circle into a quarter circle and a right triangle.
   - The area of the quarter circle is:
     \[
     \frac{1}{4} \cdot \pi \cdot r^2 = \frac{1}{4} \cdot \pi \cdot 5^2 = \frac{25\pi}{4}
     \]
   - The area of the right triangle \( \triangle OAB \) is:
     \[
     \frac{1}{2} \cdot OA \cdot OB = \frac{1}{2} \cdot 5 \cdot 5 = \frac{25}{2}
     \]
   - Therefore, the area of the larger segment is:
     \[
     \frac{3}{4} \cdot \pi \cdot 5^2 + \frac{25}{2} = \frac{75\pi}{4} + \frac{25}{2}
     \]

5. **Calculate the volume of the larger piece of the cylinder:**
   - The volume of the larger piece is the product of the height and the area of the larger segment:
     \[
     20 \cdot \left( \frac{75\pi}{4} + \frac{25}{2} \right)
     \]
     \[
     = 20 \cdot \left( \frac{75\pi}{4} + \frac{50}{4} \right)
     \]
     \[
     = 20 \cdot \frac{75\pi + 50}{4}
     \]
     \[
     = 5 \cdot (75\pi + 50)
     \]
     \[
     = 375\pi + 250
     \]

6. **Identify the values of \( a \) and \( b \):**
   - Here, \( a = 250 \) and \( b = 375 \).

7. **Sum \( a \) and \( b \):**
   \[
   a + b = 250 + 375 = 625
   \]

The final answer is \(\boxed{625}\).