A common way to check your own heart rate is to put your index and middle finger on your neck next to the windpipe, count the beats you feel over 15 seconds, and multiply that count by four. The beats per minute (BPM) you get this way is only an estimate. In general, if you count b beats in p seconds, the BPM is 60b/p.
In this problem the heart always beats at a constant interval. If t is the time in seconds between one beat and the next, the Actual Beats Per Minute (ABPM) is defined as 60/t.
A measurement takes a window of exactly p seconds and counts the beats that fall inside it. Even when the count is b, more than one value of t produces that count, so the ABPM is not determined. The ABPM values consistent with counting b beats in p seconds form an interval.