Another approach to #2 . . .
The man walks north from at 5 ft/sec.
Since he has a 5-minute (300-second) headstart.
. . he has already walked: feet.
In the next seconds, he walks feet more to
The woman walks south from at 4 ft/s.
In the next seconds, she walks feet to
The diagram looks like this:
| * R
P *---------------*-------* Q
: * |
4t: * |4t
: * |
C + - - - - - - - - - - - * B
: 1300 :
The distance between them is:
From the right triangle , we have:
. . and we have: .
"20 minutes after the woman starts walking" means: seconds.
. . Plug it in and simplify . . .