2020 AMC 12B Problem 10

Let $A = (0, 0),$ $B = (1, 0),$ $C = (1, 1),$ $D = (0, 1).$ The inscribed circle has center $\left(\tfrac12, \tfrac12\right)$ and radius $\tfrac12,$ touching $\overline{CD}$ at $M = \left(\tfrac12, 1\right).$ Line $AM$ is $y = 2x.$ Substituting into $\left(x - \tfrac12\right)^2 + \left(y - \tfrac12\right)^2 = \tfrac14$ gives $20x^2 - 12x + 1 = 0,$ with roots $x = \tfrac12$ (point $M$) and $x = \tfrac{1}{10}$ (point $P$). So $P = \left(\tfrac{1}{10}, \tfrac15\right)$ and $AP = \sqrt{\left(\tfrac{1}{10}\right)^2 + \left(\tfrac15\right)^2} = \frac{\sqrt5}{10}.$ Thus, the correct answer is B.