Two ships meet mid-ocean, and then, one ship goes south and the other ship goes west, both travelling at constant speeds

Two ships meet mid-ocean, and then, one ship goes south and the other ship goes west, both travelling at constant speeds. Two hours later, they are 60 km apart. If the speed of one of the ships is 6 km per hour more than the other one, then the speed, in km per hour, of the slower ship is

1. 12
2. 18
3. 20
4. 24