My friend sets out walking at a speed of 3 miles per hour. I set out behind her 5 minutes later at 4 miles per hour.
For how many minutes will my friend have been walking when I catch up to her?
Your friend has a head start 3 m/hr 5/60 = 1/4 mile
at the difference in your speeds 4 - 3 = 1 mph
it will take 1/4 mile / 1 m/hr = 1/4 hr (15 minutes ) to catch up to her... PLUS the 5 minutes she was walking before you started = 20 minutes