Define mutually externally tangent circles $\omega_1$, $\omega_2$, and $\omega_3$. Let $\omega_1$ and $\omega_2$ be tangent at $P$. The common external tangents of $\omega_1$ and $\omega_2$ meet at $Q$. Let $O$ be the center of $\omega_3$. If $QP = 420$ and $QO = 427$, find the radius of $\omega_3$.

[i]Proposed by Tanishq Pauskar and Mahith Gottipati[/i]
1. **Define the problem setup:**
   - Let $\omega_1$, $\omega_2$, and $\omega_3$ be mutually externally tangent circles.
   - $\omega_1$ and $\omega_2$ are tangent at point $P$.
   - The common external tangents of $\omega_1$ and $\omega_2$ meet at point $Q$.
   - Let $O$ be the center of $\omega_3$.
   - Given $QP = 420$ and $QO = 427$, we need to find the radius of $\omega_3$.

2. **Identify key points and relationships:**
   - Let $\omega_1$ and $\omega_3$ touch at point $A$.
   - Let $\omega_2$ and $\omega_3$ touch at point $B$.
   - Let one of the common external tangents of $\omega_1$ and $\omega_2$ intersect $\omega_1$ at $D$ and $\omega_2$ at $C$.
   - By angle chasing, we can show that $ABCD$ is a cyclic quadrilateral.

3. **Use the Radical Axis Theorem:**
   - The radical axis of circles $(ABP)$, $(CPD)$, and $(ABCD)$ implies that $Q$ lies on $AB$.
   - The power of point $Q$ with respect to $\omega_3$ is given by $QA \cdot QB = QP^2$.

4. **Calculate the power of point $Q$:**
   - Given $QP = 420$, we have:
     \[
     QP^2 = 420^2 = 176400
     \]

5. **Use the distance formula to find the radius of $\omega_3$:**
   - Let $r$ be the radius of $\omega_3$.
   - The distance $QO$ is given as 427.
   - Using the Pythagorean theorem in the right triangle $QPO$, we have:
     \[
     QO^2 = QP^2 + r^2
     \]
     Substituting the known values:
     \[
     427^2 = 420^2 + r^2
     \]
     \[
     182329 = 176400 + r^2
     \]
     \[
     r^2 = 182329 - 176400
     \]
     \[
     r^2 = 5929
     \]
     \[
     r = \sqrt{5929} = 77
     \]

The final answer is $\boxed{77}$.