Question 849317
Notation is important.  Do you want to express the ratio as 1.15, or as 1:15?  These are unequal.  The ratio is as, Model:Actual, ratio of model length to actual length.  This is judged to be the 1:15 based on the small size of 3 CENTIMETERS for the model's diameter.


You want this proportion:
{{{3/u=1/15}}}  using u as the actual diameter of the wheel in cm, and is unknown but solvable.