Half-Life Formula:
Amount remaining = (Initial amount)[ (1/2)^(time / half-life) ]
Amount remaining = ?
Initial amount = 200 mg
Time = 2 hours = 120 minutes
Half-Life = 20 mins
---Notice that I changed the time from hours to minutes. The time and half-life time need to be in the same unit.
A = (200)[ (1/2)^(120/20) ]
A = (200)(1/2 ^6)
A = (200)(1/64)
A = 3.125
Answer: 3.125 mg remain
Hope this helps!