I'm not to familiar with km but if it took you 30 minutes to travel 20km you would have to be going 40kph. It's basic math too. If you went 20 kph for 20km it would take you an hour to get there, but since they made it in 30 minutes, you just double the speed.
Substitute 6 into place and solving the answer gives you 4 which is indeed bigger than 3. All the other numbers will give you an answer of something lower than 3 which is not what we're looking for in this case.
