HMMT November 2009 U35

Call an integer $n>1$ radical if $2^{n}-1$ is prime. What is the 20 th smallest radical number? If $A$ is your answer, and $S$ is the correct answer, you will get $\max \left(25\left(1-\frac{|A-S|}{S}\right), 0\right)$ points, rounded to the nearest integer.