the half life radium-222 is 4 days. if a sample of radon-222 undergoes decay for 12 days, how much of the original parent isotope will be left?

Answer 1

You've made a typo. Do you mean Radium or Radon? I'll assume Rn-222.
k = 0.693/t_1/2
Then ln(No/N) = kt.
Substitute and solve for N