Jesse rode his bike 5 miles during the last half hour.
Explanation
Suppose, the constant speed of the bike is [tex]x[/tex] miles/hour
Jesse traveled 15 miles for the first 1.5 hours. As [tex]Speed= \frac{Distance}{Time}[/tex] , so the speed of the bike will be: [tex]\frac{15}{1.5} =10[/tex] miles/hour
Now also, [tex]Distance= speed*time[/tex]
So, the distance traveled for the last [tex]\frac{1}{2}[/tex] hour will be: [tex]10*\frac{1}{2}= 5 miles[/tex]