Question 267565
The Jack equation is
{{{20320 - 4x}}}
The Jill equation is
{{{0+ 1x}}}
---set them equal as
{{{1x = 20320 - 4x}}}
add 4x to both sides to get
{{{5x = 20320}}}
divide by 5 to get
{{{x = 4064}}} seconds
They meet at 
4064 feet