note the relationship between linear speed, , of a point on the edge of a circle, and its angular speed, ...

the pedal sprocket and wheel sprocket move with the same linear speed as the chain that connects them ...

this angular speed of the wheel sprocket in rpm needs to be converted to radians per hour ...

the tire and the wheel sprocket spin at the same angular speed, therefore the linear speed of the back tire is ...

I'll leave it to you to convert this linear speed into mph.