Good advice, JJ, here's an alternative that works for me.
What you have to do is look at the numbers in the problem, and find a sum that is easy to do from a times table. Then you adjust the answer from your easy sum to get close to the result. The real knack in this is getting the feel for how much this adjust ment should be.
Take our 190kts, 30nm example as before. An easy sum here is:
30 x 6 = 180
so at 180 knots we do 6 lots of 30nm per hour. So how many minutes for one 'lot':
1 hour = 60 mins = 10 mins
------ -------
6 6
So at 180 knots we will take 10 minutes to do 30nm.
But we're really going at 190 knots so we need to take a bit off the time... it's only 10 knots faster out of 180 knots, and we have ten minutes to play with, so let's estimate a figure to subtract. 0.5 look like it will be near enough:
10 minutes approx - 0.5 minutes guessed adjustment = 9.5 minutes!!!
If you want to be really clever about working out the adjustment you can see that I did a simple ratio in my head to get the adjustment 'n':
10 knots = n minutes adjustment
--------- --------------------
180 knots 10 minutes approximation
n = 10 x 10 = 10/18 ~ 0.5!!!
-------
180
Do a few of these and you get a feel for the numbers, and when you can approximate.
[This message has been edited by foghorn (edited 23 February 2001).]