Question 177441
a.)If you start with 100% and the half life is 5 hours
{{{N(t)=N[0]*e^(-alpha*t)}}}
where 
{{{alpha=ln(2)/t[hl]=ln(2)/5=0.1386}}}
Then for t=24 hours,
{{{N(t)=N[0]*e^(-alpha*t)}}}
{{{N(24)=N[0]*e^(-0.1386*24)}}}
{{{N(24)=100*0.0359}}}
{{{N(24)=3.59}}}
3.59% remains at 24 hours.
b.)Start with 100%. Find t when 90% will be gone, 10% will remain.
{{{N(t)=N[0]*e^(-alpha*t)}}}
{{{10=100*e^(-0.1386*t)}}}
{{{e^(-0.1386*t)=0.1}}}
{{{-0.1386*t=ln(0.1)}}}
{{{-0.1386*t=-2.303}}}
{{{t=-2.303/0.1386}}}
{{{t=16.6}}}
After 16.6 hours, there will be 10% left.