Hi there people, need help again! with simple stuff like these!

Here's the first set of questions from a math book...

The diamter of a motorcycle wheel is65cm

a) Work out the distance, inmetres, the motorcycle travels when the wheel makes100complete turns.

: For this one I think worked it out and here is what I have got...

65 x PI = 204.203.. x 100 = 20420.35.. ÷ 100 =204.2 (please correct me if wrong

For the next question I really got confused:::

b) Work out the number of complete turns the wheel makes when the motorcycle travels 500m.

.... confused SO MUCH, how do I work this OUT! PLEASE HELP!

I understand all the stuff about circumference, PI, radius, diameter, area, semi-circle, but when I come upon to these question of car tyres etc. I really get confused, any tips and ideas on how to mastermind this out???

THANKYOU! FOR YOUR HELP!