Hi there I have a couple questions I could not solve, if some one could help that would be great

1) Find the half-life if 2000g of mathusiasm decays to 1400g in 80 weeks. Round your answer to nearest week.

-- The formula i get is this 12log(0.7)/log(1/2)=K and my answer is wrong.

2) Find the half-life if 1000g of tspadium decays to 700g in 3 months. Round your answer to the nearest week.

3) The half-life of luvinmathism is 8 days. Write the half life equation that shows the percent, P of luvinmathism in math 12 students after t days.

Thanks in advance.