Potassium-42 has a half-life of approximately 12 hours. If a sample of ⁴²K₁₉ has an initial mass of 50. g, what mass of ⁴²K₁₉ remains after 48 hours?

a) 6.25 g
b) 12.5 g
c) 25.0 g
d) 50.0 g