So the way combat calculations work is that you have AT power and Armor defense. If AT power > Armor Defense, the guns always penetrate the armor if they hit. We can literally ignore armor pen checks. Thus, the question of whether the T72s will be killed or not only deals with two things. First, the accuracy of the javelins. Second, the value of defense of the tanks.
Accuracy depends on terrain and targeting system. Generally, it's 50% chance to hit for AT weapons with no targeting systems in plains. Enhanced targeting system is 65% I think, and enhanced II is 75% chance to hit. It's in the manual.
Second part is defense and AT value, which determines how many javelins can spot and shoot at the enemy tank. You'd think all 20 would be firing, but not all of them can see a T-72, generally, although in this case, I'm pretty sure all 20 get to fire, because 55/3 > 1.
So it's a flat 0.75*20, which equals 15. That's the same value as what you have in the game.
How would you avoid this scenario? Higher defense. More infantry means you have higher defense. Higher defense means the ATGMs can't spot the tanks because there's infantry in the way. If you had a defense of 100, you'd save ~4 tanks. If you have a defense of 200, you only lose ~8. Percent of attacks that go through is calculated by this formula:
Attacker stats go on top, defender stats go on bottom.
Where q and m are your morale (proficiency, supply, and readiness), a is attack power (you use AT power for that), j is defense, and u and o are other modifiers from terrain and entrenchment. Cities and forts multiply defense by 5 I think, and entrenchment does something. I never tested entrenchment.
Finally, you multiply this by accuracy, and then you have approximately what the game should spit out at you.
I'm gonna pass out right now, I pulled an all nighter relearning this stuff off my very poorly kept notes. I'm prepping to use my knowledge to break the game.