What size driven pulley for 200 rpm from an 8.5 inch pulley at 340 rpm?

[Janice12]

I have a 8-1/2 inch drive pulley turning at 340 rpm. I want to have it drive a pulley and want that pulley, the driven pulley, to turn at 200 rpm. What is the formula to figure out what size pulley I need. Please help

 

[brewnog]

The product of speed and diameter will be the same on both driven pulleys.