1. Ratio r: 2:1 RPM: 8 t: 20:10 R: 8:16 RPM: 16 Teeth: 20 Radius: 2

2. 24 teeth
120 teeth : :
Which takes longest to complete a turn?
Which has the greatest rpm?
The large cog turns once in a minute.
How many turns does the small cog make in this time?
5 : 1

3. 0.8 : 0.2 : 0.3 4.2 : ? : ? Radius = 0.8m Radius = 0.2m
4.2 seconds per full turn
? seconds per full turn
Radius = 0.3m
0.8 : 0.2 : 0.3
4.2 : ? : ?
? seconds per full turn

4. 0.8 : 0.2 : 0.3 4.2 :1.05: ? Radius = 0.8m Radius = 0.2m
4.2 seconds per full turn
1.05 seconds per full turn
Radius = 0.3m
0.8 : 0.2 : 0.3
4.2 :1.05: ?
? seconds per full turn

5. 0.8 : 0.2 : 0.3 4.2 :1.05:1.575 Radius = 0.8m Radius = 0.2m
4.2 seconds per full turn
1.05 seconds per full turn
Radius = 0.3m
0.8 : 0.2 : 0.3
4.2 :1.05:1.575
seconds per full turn

6. To calculate rpm from turn time:
If a wheel takes 30 seconds to make one turn, it is running at a speed of 2 rpm (it turns twice in one minute)
If it takes 5 seconds to turn, the speed is 12 rpm
To calculate rpm from turn time:
60 ÷ time