D = total trip distance
O = groundspeed out
H = groundspeed home
E = safe endurance
x = distance to PNR/PSR
c = distance to ETP
For ETP
c/H = (D-c)/O
Oc = H(D-c)
Oc = HD-Hc
c(O+H) = DH
c = DH/(O+H)
For PNR
x/O + x/H = E
Hx + Ox = EOH
x(H+O) = EOH
x = EOH/(O+H)
time to PNR/PSR = x/O = EH/(O+H)