Farmers near Raymond, Alberta, use a wheel line irrigation system to provide water to their crops. A pipe and sprinkler system is attached to a motor-driven wheel that moves the system in a circle over the field. The first wheel is attached 50 m from the pivot point, and all the other wheels are attached at 20 m intervals further along the pipe. Determine the circumference of the circle traversed by wheel 12.

I don't even understand the question or where to start, I know I have to use this formula:

Tn = T1 + (n-1)d