Question 1129811
{{{D(h)=7e^(-0.3h)}}}


h if D(h)=1:


{{{ln(D)=ln(7e^(-0.3h))}}}

{{{ln(D)=ln(7)-0.3h*ln(e)}}}

{{{-0.3h=ln(D)-ln(7)}}}

{{{h=(-1/0.3)(ln(D)-ln(7))}}}

substitute
{{{h=(-1/0.3)(ln(1)-ln(7))}}}

{{{h=(1/0.3)(ln(7)-ln(1))}}}

{{{highlight(h=6.5)}}}
6 hours 30  minutes
OR
{{{6&1/2}}}{{{hours}}}