Combat doesn't work like that though TMIT.
Combat strength is average of current HP and max HP.
The average of current and max HP is only used for the damage calculation in each combat round. For the probability of winning a round, only current HP gets used. Because the tank only has 50 HP, it will survive far less rounds, even if it
does more damage in a single round.
Tank, str 28. 50 HP. strength of the tank for damage calc 28 * 0.75 = 21
Cav str 15 full health.
chance for the cav to win a combat round = 15 / (15 + 28 * 0.5) = 51.7%
Damage done by tank in one round 20 * ( 3*21+15)/(3*15+21) = 23 HP
Damage done by cav in one round 20 * (3*15+21)/( 3*21+15) = 16 HP
The tank needs 5 rounds to kill the cavalry, but the cavalry only 4 rounds to kill the tank, because the tank starts out with only 50 HP.
Overall chance to win ~65%
If the cavalry has one extra promo, it can kill the tank in 3 rounds and winning
odds increase to about 80%