Monkeynuts26
These are the sort of sums that pilots and WSOs need to be able to do in their sleep. By the end of flying training, if you can't do these sort of sums instinctively - well, I doubt you will be at the end of flying training!
One small point though, I'm not sure what the OASC questions are like nowdays, but I assume they are semi realistic. Aircraft speeds are ususally kept to some function of 6 or thereabouts, ie 600kts, 540kts, 480kts, or in the case of a Herc, 300 kts, 240kts, 210kts, 180kts. It is also helpful to know how fast these speeds are in miles (nautical miles) per minute, ie 4 mpm, 3 1/2 mpm, 3 mpm.