It doesn’t look like there’s enough context to 100% say for sure.
Average collection period formula is for the entire year:
average collection period=receivables still duetotal credit sales for the year×365
So extrapolating that formula for intervals as per the example you get, e.g. for <31 days:
average collection period (<31)=receivables still due for <31 daystotal credit sales within last 31 days×31
Unless that info is given (credit sales for the various intervals) I’d say that’s just a given value for the purposes of that specific example. Unless maybe there’s more info before the bit you included here.