Question 717922
<pre>
There are 60 minute-marks around the face of a clock.

The speed of the minute hand is 60 minute-marks per 1 hour,

or {{{(60_minute_marks)/hr}}} or {{{60}}}{{{minute_marks/hr}}}

The speed of the hour hand is 60 minute-marks per 12 hours.

or {{{(60_minute_marks)/(12_hours)}}} or {{{60/12}}}{{{minute_graduations/hr}}} or {{{5}}}{{{minute_graduations/hr}}}

At 4:00 the hour hand has a 20 minute-mark head start on the hour
hand, so the minute hand's catch-up rate is 60-5 or {{{55}}}{{{minute_marks/hr}}}.  
Since time = {{{distance/rate}}}, the minute hand will catch up to the hour hand
in {{{20/55}}} hr which reduces to {{{4/11}}} of an hour which is {{{expr(4/11)*60}}} minutes
or {{{240/11}}} minutes, or {{{21&9/11}}} minutes

That's  21 minutes and {{{49&1/11}}} seconds. 

Edwin</pre>