Assuming first order kinetics what fraction of an IV drug dose remains in the body after 4 half-lives?

PHARMACOKINETICS PROBLEMS
1.    Assuming first order kinetics what fraction of an IV drug dose remains in the body after 4 half-lives?
2.    The t1/2 of a drug is 6 hours. What fraction is left in the body at 21h post drug administration?
3.    Twenty percent of a drug dose remains in the body at t=10hours. What is the half-life of the drug?
4.    The plasma concentration of a drug administered by IV bolus injection may be described by the following equation:
Ct = 0.92e -0.35t
where Ct is in units of mg/L and the time (t) is in hours.
If the dose is 50mg calculate the following:
(a)    The plasma concentration at 6.5h post dosing.
(b)    The plasma half-life.
(c)    The volume of distribution.
(d)    The area under the plasma concentration versus time curve.
(e)    The total clearance.