Question 29056
{{{t =  sqrt( d^3) / 216}}}
[a] {{{t = sqrt(8^3) / 216}}}
{{{t = sqrt(8*8) * sqrt(4*2) / 216}}}
{{{t = 8 * 2 * sqrt(2) / 216}}}
{{{t = 2 * 1.414 / 27}}}
{{{t = .1047}}}
This is in hours
for minutes and seconds
.1047 hours x 60 minutes/hour = 6.284 minutes
.284 minutes x 60 seconds/minute = 17 seconds
so the time is 6 minutes 17 seconds
---------------------------------
twice the diameter would be 16 miles or 2*8 miles
{{{t =  sqrt( d^3) / 216}}}
{{{t = sqrt((2*8)^3) / 216}}}
{{{t = sqrt(2^3*8^3) / 216}}}
that's because  (2*8) * (2*8) * (2*8) = (2*2*2) * (8*8*8)
[b] {{{t = sqrt(2^3) * sqrt(8^3) / 216}}}}
notice the difference between [a] (the 2nd line) and [b]
the difference is the factor {{{sqrt(2^3)}}}
so if you double the diameter this is the factor you multiply the time by 
to get the new time.