The thing to remember about nuclear half-life calculations is that on original sample of a radioactive substance will be halved with every passing of a half-life.
Mathematically, you can write this as
#color(blue)(A = A_0 * 1/2^n)" "#, where
Keep in mind,
In your case, the mass of the original sample is
#color(blue)(n = "total time"/"half-life")#
In this case, you will have
#n = (50.0color(red)(cancel(color(black)("years"))))/(12.3color(red)(cancel(color(black)("years"))))#
This means that you'll be left with
#A = "1.00 g" * 1/2^(50.0/12.3) = color(green)("0.0597 g tritium")#
The answer is rounded to three sig figs.