Am I doing battle odds right?

I'm driving a 800 plus hit point army smashing 150 against infantry, 125 against armour, and 75 against heavy.

I'm running into a 110 hit point 5 army group composed of 2 15 hitpoint infantry, 2 20 hp armour, and 1 40 hp heavy.

I figured divide 150 by 5 for infantry, 125 by 5 for armor, and 75 by 5.

So the armies run into each other and his medium tank is gone.

I would have thought I would be facing 40 -15 or a 25 hitpoint tank still standing there. I blew right through it.

Is the overkill on other armies transferred over to remaining. Like artilery strike?