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I wish there was an easy answer to that Q - there are posts here (if you search) on how the rewards are calculated but the $80 is an average of the value of steem over the past 3 days I think. It's then split into steem and sbd. The benefit though is that the sbd is treated as $1 when they're actually worth closer to $3 at the moment. You can find latest values at coinmarketcap.com. I wish it was easier to explain!

You choose if you want 50/50 SBD/SP or 100 SP on any post you make. If you want to keep liqiud assets SBD you should choose 50/50 SBD/SP which will mean that in 7 days after you posted your content you will get half in SBD (half the dollar figure you see by your post), the other half will be in SP (calculated at the current valuation of STEEM converted into SP (Steem Power).

The other option 100 SP means your post would be payed out ONLY in SP which is great if you are in for the long haul. You can change for each post if you wish, It's good to use both, just try 50/50 on a post and try 100 percent SP on another, that way you can have both liquid assets and build SP too. Hope that is helpful to you.

Don't try to think about it too much other than 'It's magical internet money' that can also let you buy into other 'magical internet money' such as Bitcoin, Ethereum, Dash, Litecoin etc if you don't want to keep it all as STEEM/SBD SP (diversify your portfiolio). For now be happy you are noticed by people, or bots, or whatever is upvoting. Enjoy.

Just remember, the dollars you see under your posts are NOT US Dollars, they are STEEM Token Units (STU). These vary in price according to market pressure. If you see $80 under your post it ISN'T US Dollars. The reward you get depends upon how you chose to accept your rewards (chosen when you make the post, either 50/50 SBD/SP or 100 percent SP).