how many moles are in 25.0 grams of potassium?
1 mol Potassium = 39g [Atomic Mass of K]
x mol Potassium = 25g
x = 25*1/39 = 0.641 mol
Hope that helps.
I would remove the 1 and make the answer 0.64 since there are only two significant figures in 25 and 39.