Multiple replies again so look for your name....
Abz217: No-one seems to know what the 'pass' rate is or if you're marked for how many you got right out of the number you answered (accuracy), or how many you answered and how many you got right (accuracy and speed). This thread has much speculation on this matter - my best advice would be to answer as many as you can correctly and hope you pass... Tests are all done on paper with a question booklet and a separate multiple choice answer sheet to be filled in.
Gogz87: I got a chance to visit Oceanic Control at Prestwick when I went for my visit so hopefully this makes sense! I can't believe Mach number is 'more accurate' than any other unit of velocity since they are all related (and can be interconverted). However, it's easier to sort out the spacing of aircraft over the ocean when using Mach number: the minimum longitudinal separation for aircraft on the same track is 10 minutes flying time. If two aircraft are going at exactly the same speed, if aircraft 1 enters the track system at 1410 and aircraft 2 enters at 1420, everything will be fine for the duration of transit across the ocean.
However, if the speeds are different, you need to be able to calculate what gap needs to be in place at the start of the journey to ensure there is still at least 10 minutes between the aircraft at the end. You do this as follows (I've stolen this from a good page on the internet about Shanwick/Gander operations -
Gander / Shanwick Oceanic - IVAO):
Slow followed by fast: Add 1 minute to the standard for every increase of 0.01 Mach number of the second aircraft.
Example: M0.80 followed by M0.84 requires FOURTEEN minutes at ocean entry same track same level.
Fast followed by slow. Subtract 1 minute from the standard for every decrease of 0.02 Mach number of the second aircraft. The minimum is 5 minutes at Oceanic entry.
Example: M0.84 followed by M0.80 requires a minimum of EIGHT minutes separation at ocean entry same track same level.
Hope this answers your question...