Can anyone help on this chemistry problem?

Question

Part A

If 125. mg of fluorine-18 is shipped at 7:00 A.M., how many milligrams of the radioisotope are still active after 110 min ?

Express the mass to three significant figures and include the appropriate units.

Part B

If 200. mgmg of fluorine-18 is shipped at 7:00 A.M., how many milligrams of the radioisotope are still active when the sample arrives at the radiology laboratory at 4:10 P.M.?    

Express the mass to three significant figures and include the appropriate units.

Answer 1

A: We know the half-life of fluorine-18 is about 110 minutes. Therefore, by the definition of half-life, half of the mass will remain after one half-life. Therefore, 125/2= 62.5mg of fluorine-18 will remain.

B: Between 7:00 AM and 4:10 PM, 9 hours and 10 minutes will have passed. Converting this to minutes, we get 550 minutes. Therefore, this represents 550/110 = 5 half lives. Therefore, you can multiply the original amount by .5 five times (200*.5*.5*.5*.5*.5) which will give you the final amount of mass remaining after five half-lives have passed.