Question 352503
Rate*Time=Distance
.
.
.
{{{R[a]*t=D[a]}}}
{{{D[a]=78t}}}
.
.
{{{R[b]*t=D[b]}}}
{{{D[b]=60t}}}
Convert 500 feet to miles to be consistent.
{{{D[a]=D[b]+500/5280}}}
Substitute and solve for {{{t}}}.
{{{78t=60t+25/264}}}
{{{18t=25/264}}}
{{{t=25/4752}}} hrs
or approximately, 
{{{t=0.00526}}} hrs
{{{t=18.9}}} seconds