hmmm... not really any shortcuts, but doing such sums repeatedly in your head does help eventually (I know it did for me when I went to OASC).
For the 90mph 50 mile question, and referring to the fact that 60mph is 1 mile per minute, you are effectively doing 90/60 miles per minute, or 1.5. Divide 50 by that (which is the same as multiplying by 2/3) and you get 33mins and 20 secs.
Alternatively you could divide 50 by 90 and multiply that by 60.
Sometimes fractions are a better way of getting a near (if not totally correct) answer... At least that's what I find.
Work through some examples and find a system that best suits you.
Gosh I sound like my maths teacher!
Hope this helps, all the best with OASC
Hungry
EDIT:
BEagle just saw your post...glad to see I'm not too rusty