How would you go about solving this? (it's question 13 in the image)

The decay of uranium is modelled by D=D₀ x 2^-kt. If it takes 6 years for the mass of uranium to half, find the percentage remaining after

a) 2 years

b) 5 years

c) 10 years

The answers are a) 79%, b) 56% and c) 31%