Which of the following is a prime number? 
Overseas Competition->Knowledge Point->Number Theory Modules->Prime and Composite Numbers->Determining Prime and Composite Numbers
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. 