You would be left with 1.25 g.
After 0.001 second, 10 grams decays to 5 grams.
After another 0.001 second, 5 g decays to 2.5 g. (Total time 0.002 s; two half-lives have passed)
After another 0.001 second, 2.5 g decays to 1.25 g. (Total time 0.003 s; 3 half-lives have passed)
