In situations like this it can be helpful to look at what the equations would yield.
In this case we are looking for the time to PNR and time is measured in hours.
Endurance is also measured in hours. Ground speed is measured in knots which are NM/hour
So in the equation we have PNR = E*H / O*H
Hours to PNR = (Hours x NM/hour ) / (NM/hour x NM/hour)
Which simplifies to give:
Hours to PNR = NM / ( NM squared x hours squared )
Dividing top and bottom by Nm gives:
Hours to PNR = 1 / (NM x hours squared )
Multiplying top and bottom by hours squared gives:
Hours to PNR = ( Hours squared / NM )
I don’t know what an hours squared/NM looks like, but it is clear that hours and hours squared / NM are not the same thing. So this first equation does not yield a time in hours.
Now let’s try the second equation
PNR = E*H / O + H
Hours to PNR = ( Hours x NM/Hour) / (Nm/hour + NM/hour )
This simplifies to give:
Hours to PNR = ( Hours x NM/Hour) / (Nm/hour )
Dividing top and bottom by NM/hour gives
Hours to PNR = ( Hours ) / ( 1 )
Multiplying the top and bottom by 1 gives;
Hours to PNR = Hours
So this second equation does yield an answer that is in the correct units (hours).
The process as written above looks a bit complicated, but this is because I have explained each step. But in practice it is easy provided you are reasonably good at maths. If you are not reasonably good at maths it will take a bit more effort, but the practice will do you good.