Question 200523
{{{Distance = Rate*Time}}}
Let David's time be given as t.
Then Carl's time is t-30.  
{{{DavesDistance = 18*t}}}
{{{CarlsDistance = 20*(t-30)}}}
When Carl catches Dave, their distances are the same
{{{18t = 20t - 600}}}
{{{-2t = -600}}}
{{{t = 300}}}  
that 300 is in minutes

So 5 hours in, they are both 18*300 = 20*270 = 5400 feet on their way.
FYI - climbing a mile in 5 hours is 'hiking'

Your answer is correct as far as you took it. You just have the units wrong. Your answer is time ... not distance. Your answer says Carl was hiking for 270 minutes .. not 270 feet.
Dave left 30 minutes earlier, so he has walked 300 minutes (same as my answer)

To convert time to distance, use {{{Distance = Rate * Time}}}