Exchange rate is 2 guilders for every 3 florins
The conversion factor will be
2 guilders / 3 florins = 3 florins / 2 guilders = 1
so if we wish to convert 17 guilders to florins we will use following conversion factor
3 florins / 2 guilders = B / C
D = 17guilders X 3 florins / 2 guilders = 25.5 florins