The transformation is not possible. The cup is a unit for volume, not for mass. You need the density of the material. The metric cup is eqivalent to 250 mL; in different countries other volumes are valid.

How many cups are in 420 grams depends on the product in question. For instance, 420 grams of sugar is nearly equal to 2 cups while 420 grams of coconut is roughly 4 cups.

225 grams of flour is .95 cups. The formula to convert grams to cups is 1 gram = 0.00423. So, you would multiply how many grams you have by 0.00423 to find out how many cups you need.