Collision avoidance math?

[flydaddy]

Hi all,

Somebody give me a shove with the simplist way of calculating impact times.

IE. 2 aircraft head on at 550 k see each other at 4 kilometers, now calculate how long until impact.

I realise I need to convert 4 kilometers to NM by multiplying by .54 but from there I go blank.

At one point I could do this but now cannot.

Please help a mental blocked getting annoyed oldtimer.

Thanks.

Happy Landings

Flydaddy.