Lets draw a picture of the penny-farthing bicycle:
where x is the diameter of the back wheel. Since the ratio is 3 to 1, we have

By moving 48 to the right hand side, we have

This means that the diameter of the back wheel is 16 inches. Now, the circunference formula is

where d is the diameter. Then, the circunference of the back wheel is

What is the circumference of the back wheel? 50.24 inches