Question 316548
{{{h(N)=256*(1/2)^N}}}
Find N so that 
{{{h(N)<15}}}
{{{256*(0.5)^N<15}}}
{{{(0.5)^N<(15/256)}}}
{{{N>log(0.5,(15/256))}}}
{{{N>log((15/256))/log((0.5))}}}
{{{N>4.093}}}
On the 5th bounce, it will be below 15 feet.