In your interview you will have to do one off the top of your head. Everybody recommends that when you do this you should speak your thinking out loud so that the interviewer can at least see that you haven't just frozen up.

You have probably already seen this link but, just in case you haven't, I'll put it up because it is indispensable:

Speed Distance Time | OASC: RAF Officer and Aircrew Selection Centre
Read the advice on that page (the one in the link) carefully.

Learn the divisions of an hour:

2 of 30 minutes 1/2 hour = 30 mins

3 of 20 minutes 1/3 hour = 20 mins, 2/3 hour = 60 mins

4 of 15 minutes 1/4 hour = 15 mins, 3/4 hour = 45 mins

5 of 12 minutes 1/5 hour = 12 mins, 2/5 hour = 24 mins, 3/5 hour 36 mins, 4/5 hour = 48 mins

6 of 10 minutes 1/6 hour = 10 mins, 2/6 hour = 20 mins...

10 of 6 minutes 1/10 = 6 mins

12 of 5 minutes 1/12 = 5 mins

15 of 4 minutes 1/15 = 4 mins

20 of 3 minutes 1/20 = 3 mins

30 of 2 minutes 1/30 = 2 mins

These quickly become second nature, so when presented with a Speed question like:

"What speed covers 6 miles in 24 mins?"

You already know that 24 mins is 2/5 of an hour. Therefore 6 miles is 2/5 of the speed

(You covered 6 miles in 2/5 of an hour, so your speed is 6 miles per 2/5 of an hour).

So work out what it is for 5/5 of an hour:

6 = 2/5 of an hour

3 = 1/5 of an hour

15 = 5/5 of an hour ---> 15 miles per hour

Here's a slightly bigger example:

"What speed covers 288 miles in 9 hours and 36 mins?"

The "36 mins" is the key clue. 36 mins is 3/5 of an hour, so you know you're working in fifths of hours.

36 mins = 3/5 of an hour

9 hours and 36 mins = 48/5 of an hour

Therefore, 288 miles = 48/5 of the speed per an hour

.... work out what 1/5 is....so that you can work out what 5/5 is:

288 = 48/5

(48,96,144,192,240,288.........288 divided by 48 = 6)

6 = 1/5

30 = 5/5..... 30 miles per 5/5 of an hour....30mph

Do the automated tests over and over and over...

You'll start to get good at the little ones and develop your own quick, logical way of doing them.

For the tricky, bigger ones:

Stop muddling yourself up trying to work it out.

Clear your mind.

Go back to the basic S

/T , T= D/S , D=SxT

Write it all out (in the plan-ex)

Say it out loud (in the interview)