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Answer 1

I'm sure you know how to evaluate the expression for t=0, t=28 and t=56.
The next parts wants to know what each value in the expression means. For this we reference the half-life formula \[N(t)=N_02^{-{t\over t_{1/2}}}\]Where N(t) is the remaining amount of material after t years.
No is the Initial amount of material
t is the time in years
t 1/2 (the bottom of that fraction) is the half-life time of the radioactive material