A drug has a half-life of 4 hours. Assuming someone takes a dose of 512 mg, how much of the drug remains in the system after 36 hours?

Question

0.0000001 mg
	168 mg
	1 mg
	0.5 mg

Answer 1

If you set up a table, this is easier to see...

If the half-life is 4 hours, than only HALF will remain after 4 hour.

Time ................ Amount in body

0 hours .......... 512 mg

at end of 4 hours 256 mg

at end of 8 hours 128 mg

at end of 12 hours 64 mg

at end of 16 hours 32 mg

at end of 20 hours 16 mg

at end of 24 hours 8 mg

at end of 28 hours 4 mg

at end of 32 hours 2 mg

at end of 36 hours 1 mg

Answer is 1 mg.

Answer 2

This would follow the rules for exponential decay where

Final amount = Initial x r^t where r= 0.5( half life) and t is 9 since 36 hours is 9 half lives

So Final = 512 x(0.5)^9= 1mg