Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

ok so i dont think the math is right
9 banshees + exarch w/ executioner
27/2=13.5 hits
13.5/3=4.5 wounds
3*2/3=2 hits
2*2/3=1.3333 wounds
4.5+1.333=5.8888 so six wounds

3 Marines + Sargent w/ PW+BP
3/2=1.5 hits
1.5*2/3=1 wounds
1/2=.5 unsaved
3/2=1.5 hits
1.5*2/3=1 wounds
.5+1=1.5 dead

if the banshee exarch has mirror swords then its 1.11 dead marines, so probably still 6 in total, if the marines are armed with CC/BP then it goes up to 2 dead banshees.
Against 10 Plague Marines its about 2 dead PMs (no charge bonus hurts) they kill 3 in return (1 PF champ) and if banshees are not run down they eventually loose
or against 7 they kill 2 on the return attacks and banshees will eventually win.
Against beserkers who ever charges wins.
Those are the 4 most common variant marines i can think of.