20 minutes is [tex]\frac{1}{3}[/tex] of an hour (60 minutes).
Stan's constant rate is r miles per hour, which means he drove r miles in 60 minutes. So how many miles did he drive in 20 minutes? As explained in the first line, 20 minutes is [tex]\frac{1}{3}[/tex] of an hour. Since his rate is constant, Stan would drive [tex]\frac{1}{3}[/tex] of what he cover in 60 minutes, which is r miles.
Hence, Stan drove [tex]\frac{1}{3}r[/tex] miles in 20 minutes.
ANSWER: [tex]\frac{1}{3}r[/tex] miles
