You have got four gear wheels connected in a row. The four gears have 30 teeth, 10 teeth, 20 teeth and 10 teeth.
If you turn the first gear (the one with 30 teeth) around 20 times, how many times does the fourth gear turn?
As the first gear has three times the number of teeth that the second gear has, the second gear will turn three times as often - 60 times.
As the third gear has twice the number of teeth that the second gear has, the third gear will turn half as often - 30 times.
As the third gear has twice the number of teeth that the fourth gear has, the fourth gear will turn twice as often - 60 times.
In fact, as the second and fourth gears have the same number of teeth, the number of teeth on the third gear is not relevant.