The middle ratio number represents the time spent at 30 mph, and the total of all ratio numbers represents the total driving time. Then the fraction spent at 30 mph is ...
4/(6+4+3) = 4/13
_____
Total driving time was 13/20 hours, or 39 minutes. Of that time, 12 minutes was spent driving at 30 mph.
The distance around a rectangle or a square is as you might remember called the perimeter. The distance around a circle on the other hand is called the circumference. It is calculated as follows:
C = 2pi(r)
From this, we can solve for r and evetually solve for the diameter.
