Lets do it manually... 320 + 320 = 640.
640 + 640 + 640 + 640 + 320 = 1280 + 1280 +320 = 2560 + 320 = 2880.
Fairly straight forward if you are into computers and are familiar with Binary... take the zero off and treat it like 32... 32 times 8 is 256... put the zero back on and add 320 and you get 2880m/s...
10 times the speed of sound at 320m/s would be 3200m/s or 3.2km/s... deduct 320m/s and you get 2880m/s... so my numbers seem to add up.
Either way covering the 400km EEZ to Washington is going to be less than 3 minutes... you could have an obvious missile carrier there 24/7 and a few less obvious also there in case that one is preemptively attacked... which in itself would be justification for the other clandestine platforms to immediately attack... so either way you get your shot at the Washington cronies.
Imagine... right now... start a stopwatch... wherever you are... what can you do in 2 and a half minutes?
Not very much... It would be very restrictive to have to remain within 2 minutes and 30 seconds from a security bunker... and when Gzur II is ready and moves even faster... at mach 12-14... well then... perhaps the US is going to have to tone down the attitude?