For the answer to the question above, the decay function is F = A(0.5)^(t/140) where A is the initial amount of Polonium-210 and F is the remaining amount after t days of decay.
So F/A = (0.5)^(425/140) = (0.5)^3.035… = 0.12194… = 12.194%
Therefore F = 12.194%
i hope this helps