gcd(12, 18) = 6, lcm = 36
gcd(7, 13) = 1, lcm = 91
gcd(100, 75) = 25, lcm = 300
gcd(270, 192) = 6, lcm = 8640
1/2 + 1/3   = 5/6
1/2 - 1/3   = 1/6
3/4 * 2/3   = 1/2
3/4 / 1/2   = 3/2
6/8 == 3/4  : yes
1/3 < 1/2   : yes
2 + 1/2     = 5/2
4/-6        = -2/3
1/2 / 0     : refused, denominator is zero
