Hey again everyone.... I’m so sorry... you are all my saviours with questions as I’m a self study gal...
But I’m doing practice exams.. not Bob... but this one... well... I have no idea...
I’ll write it out exactly as in the book...

Departure airport A
Destination airport b
TAS 140kts
Sector distance airport a to airport b is 420nm
Wind component out to pnr -40kt tail
Wind component home from pnr -40kt head
Positive fix at 0140 utc it is established in the cruise 100nm past airport a
Safe endurance 200mins
Should the need arise to divert back to airport a you may disregard all climbs and descents
The position of the pnr for airport a measured from airport a is closest to....

So I have used the formula...
Time to PNR = safe endurance x Ground speed home / 2x TAS
My choices for answers are
1.170nm
2.150nm
3.200nm
4.250nm
I’d be interested to see what everyone else calculates it as being

Standard approach is fine.

(a) figure the fuel available for PNR at the fix position in whatever is your usual manner - the only sensible consideration is that the endurance quoted is for that position. Note that you have quarantined your reserves so the calculations now are just for cruise. (Use of the term safe endurance provides an opportunity for confusion. If it means actual endurance, then fine, however, it may mean endurance after reserves have been quarantined - the answers should clarify which is the case).

(b) figure the fuel burn back to A from the fix position. This leaves you with the fuel available to plug into your radius of action (PNR) equation.

(c) remember to add the 100nm to the distance you calculate from the fix to the PNR position.

Why don't you run the sums, post your working as a scan, and we can comment. That way you will get the maximum value from the discussion.

Soph

Just remember that in the PEXO exam CASA will not give you a wind component it will be a vector problem as well as PNR calculation.