Question 110
A circle $O_1$ of radius $2$ and a circle $O_2$ of radius $4$ are externally tangent at the point $P$. Points $A$ and $B$, both distinct from the point $P$, are chosen on the circumference of the circles $O_1$ and $O_2$, respectively, in such a way that the points $A$, $P$ and $B$ are collinear. Determine the length of the line segment $PB$ if the length of $AB$ is $4$.
