Take b = 2.

b^(n-1) mod n = 1.

71 is prime.

b^((n-1)/71)-1 mod n = 1651, which is a unit, inverse 913.

61 is prime.

b^((n-1)/61)-1 mod n = 2734, which is a unit, inverse 1353.

(61 * 71) divides n-1.

(61 * 71)^2 > n.

n is prime by Pocklington's theorem.