"VA Math." Why 50% + 50% might be only 70%. Or 65%. Or it could be 100%. How's that possible?

How? The VA does not add disability percentages for each recognized service-connected disability when calculating a veteran's disability rating. Rather, each disability's rating, started with the largest (and presumably the most serious) is applied to the portion of "independent healthy working adult" that remains “healthy.”  So we can all agree that a 100% previously healthy veteran who experiences a 50% disability has a 50% total VA disability rating. 

So far, that's logical. No brain pain understanding that math.

Here's where logic is less logical but it kinda makes sense...if only from the VA's perspective: A 50% rating for one disability leaves a vet with a remaining 50% working efficiency, but another 30% added for a second disability consumes 30% of that remaining 50%, (that's .3 x .5) yielding 65% total, but that's rounded to 70% for disability compensation purposes. 

Thus, 50% plus 50% equals 70%. But only at 810 Vermont Street, Washington DC (VA HQ).

Don't keep scratching your head trying to understand. Its the VA – it just is.