# Interview Question: Sexy Primes

This is the fourth of a series of short posts in which I solve interview questions from my Professional Practices course using Ruby.

## The Prompt

Sexy prime numbers are prime numbers that differ by six. Examples would be 7 and 13, 11 and 17, and so on. Find all of the pairs of sexy prime numbers up to 500.

## The Solution

The simplest solution is to use the ‘prime’ library that is included with Ruby by default:

```
require 'prime'
(1..500).each do |num|
if Prime.prime?(num)
numPlusSix = num + 6;
if Prime.prime?(numPlusSix)
puts [num, numPlusSix].to_s
end
end
end
```

Here is another way to solve the problem without requiring any libraries:

```
def isPrime?(number)
case number
when 1
false
when 2
true
else
(2...number).each do |x|
if number % x == 0
return false
end
end
return true;
end
end
(1..500).each do |num|
if isPrime?(num)
numPlusSix = num + 6;
if isPrime?(numPlusSix)
puts [num, numPlusSix].to_s
end
end
end
```