They have scales and they rub off easily
15+3=18km/hour
Think about it like this. The boat is going 15 faster than the river, and the river is going 3 faster than the bank, so the boat is going 18 faster than the river bank
You can use mostly anything as long as it is circular. Depending on how big it is, you could use sturdy paper plates and use a stick/rod and tape to hold it together, or you could use bottle caps if the car you are trying to make is really small.
Max preassure = force / min area
= 3N / 0.1 x 0.05
= 600N/m(squared)
Copy off of the picture below itll help better, its what someone sent me when i asked this question