Which number is a prime number?
141
161
171
151

141 (The original number) ;1 + 4 + 1 = 6 (Add each individual digit together) ;6 is divisible by 3 ;Then 141 is divisible by 3 ;141 isn't a prime number ;In the same way, 171 is divisible by 9 ; then, 171 isn't a prime number ;

161 = 7 x 23; 161 is divisible by 7 ; then, 161  isn't a prime number ;

Test whether 171 is divisible by 2, 3 , 5 , 7 , 11 , 13 ( an algorithm that tests each incremental m against all known primes <  division ) ;
but, 171 isn't divisibile by 2,3,5,7,11,13 ;
Finally, 171 is a prime number ;