Circumference of a circle HELP!!!!

*Hi there people, need help again(Doh)! with simple stuff like these!*

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

The diamter of a motorcycle wheel is** 65cm**

a) Work out the distance, in **metres**, the motorcycle travels when the wheel makes **100** complete 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!