Your figures are a mixture of seconds and decimal parts of minutes, so you must take care to convert these to common units. Your figures for longitude are unclear but I have assumed both positions are West.
The N/S error is INS – Ramp = (S53°12.5') – (S53°21'30'') = -9'
This is a N/S error of 9 minutes of latitude so it is equal to a distance of 9 nm.
The E/W error = INS – Ramp = (W002°36.4') – (W002°16'24'') = +20'
This is equal to a distance (20 x Cos mean latitude) nm
Using S53°17' as mean latitude gives an E/W error distance of
20' x Cos 53°17' = 19.9 m.
Now sketch the situation in the form of a right-angled triangle with base = 19.9 nm and vertical side = 9 nm.
The total error is the hypotenuse of this triangle so we have
Error = square root of ( 9 squared + 19.9 squared) = 21.84 nm
Dividing this error by the flight time of 7.5 hours gives an error rate of
21.84 nm / 7.5 hours = 2.9 nm/hour.