US Military veterans can receive certain benefits from the VA for their service. One of these benefits is disability treatment and compensation. The amount of compensation, or the level of treatment you receive will depend on your combined disability rating. Many people become confused at the math used when combining separate disability related issues.
Let's say you have 5 rated health issues. Two are considered 20% disabling, two are considered 10% disabling, and one is considered 0% disabling. You might think you can simply add the disability ratings together for a total of 60%. That's not how it works.
Long Version: The military uses a different approach to disabilities. They look at how "abled" you are as opposed to how "disabled" you are. They start with the highest rated disability, in this case 20%. For this first rating, rather than looking at you as 20% DISabled, you are considered 80% abled. Now let's add your next highest rating of 20. Take 20% of your 80 ability rating is 16. Now you subtract 16 from 80 and get 64. You are now 64% abled. Now continue with the next rating of 10%. Mathematically, 10% of your 64% abled body is 6.4. Subtract 6.4 from 64 and you get 57.6. That's your new "abled" level. One final 10% computed in to the 57.6 and you get 5.76. Subtract that from 57.6 and you get 51.84. The final 0% is meaningless, although you can technically say it gets computed.
So with your final "abled" rating of 51.84, you are 51.84% abled. That means you are 49.16% disabled. This final disability percentage, and ONLY this final number gets rounded by standard math procedures to the nearest 10% (4 and below round down, 5 and above round up). This means you are 50% disabled, not the 60% you would get if you added your percentages together.
Note on the 0%: In case you're wondering what the purpose of a 0% rating is, this means you can get treatment for the condition at VA medical facilities, but you're just not disabled due to it.
Short Version: It's a lot easier to convert your disabled percentages to abled percentages and just multiply them. Instead of looking at 20% 20% 10% and 10%, look at it as 80% abled times 80% abled times 90% abled times 90% abled. Simplified, this is .8*.8*.9*.9 = .5184 or 51.84% abled. This, as you will notice, is the exact same number achieved above...just much simpler!
Note: There are many times where a mathematic addition of 10% results in no rating increase. Many people feel this is unfair. "How can I be 40% disabled with 10% more than when I was 40% disabled?" It's just one of those things that depends on how you look at it. If you can grasp the military version, you're not disabled...you're abled.