The amount that will remain after 3.7 hours for isotope chlorine that has a half life of 55.5 minutes is 2.813 kg
Explanation
Half life is the time required for the radio activity of a specified isotope to fall to a half its original value.
convert 3.7 hrs into minutes
= 3.7 x 60 = 222 minute
find the number of half life that chlorine isotope undergo in 222 minutes
=222 minutes / 55.5 minutes = 4 half life
therefore 45 kg after first half life =45 kg /2=22 .5 kg
22.5 kg after second half life = 22.5/2 =11.25 kg
11.25 kg after third half life = 11.25/2 = 5.625 kg
5.625 kg after fourth half life = 5.625/2 = 2.813 kg