We note that the other options are not prime numbers because:  $39 = 3 \times 13$  $50 = 5 \times 10$  $87 = 3 \times 29$  Hence by the process of elimination, $79$ is a prime number. 
Which of the following is a prime number? 

* $$39$$ 
* $$50$$ 
* $$79$$ 
* $$87$$ 