in 3 hours half of 700 would be 700/2=350
in 3 more hours, so 6 total, the half life would be 350/2 = 175
in 3 more hours, 9 total, the half life would be 175/2 = 87.5
in 3 more hours, 12 total, the live life would be 87.5 / 2 = 43.75
so it would be 12 hours to have less than 50 mg
