The general formula is M = M0/(2^(t/28)), Where M0 is the initial mass; M is the current (remaining) mass, and "t" is time in years. So, the answers (and the calculation work) are (a) M = 10/(2^(1/28)) = 9.755 grams; (b) M = 10/(2^(10/28)) = 7.807 grams.