240 students = 48% of total enrollment
48% means 0.48
"of" means "times" .
240 students = 0.48 x total enrollment
Divide each side by 0.48 :
Total enrollment = 240 students / 0.48 = <em>500 students</em>
A half-life of 9 minutes corresponds to a decay factor <em>k</em> such that

Then the time it takes for the substance to decay from 610 g to 124 g is time <em>t </em>such that

Solve for <em>t</em> :



so it takes about 20.7 min for the 610 g sample to decay down to 124 g.
FED is 5/4 times bigger than CBA. So, 44×1.25 is 55. 40×1.25 is 50. The perimeter is 125