And with these purchase prices, you can have these things paid off quickly if you concentrated on it. 

Not starting in debt and concentrating on the cash flow.

You can find every single book there is on real estate investment in amazon for kindle, usually bellow $10, and once you have the book bought and download on your kindle app, you can ask Alexa to read it for you.By listening the book while you read it on the app, you can concentrate more, retain more, and keep a longer time focus on it. 

I concentrated in duplexes and triplexes and prices rose from $100K in 1980 to $350K in 1986, 6 years.

This is the logic behind a push-pull marketing strategy.Ultimately, you'll want to have a system that attracts only qualified prospects to your funnel so that you can concentrate only on sellers with viable opportunities. 

I would concentrate on college. 

Since I'm just starting out I would like to concentrate on cash-flowing properties in C areas.

i would say that it seems the areas of highest concentration have moved a bit but then again who can predict mother nature.  

One of the areas I concentrate on most for my clients is scaling. 

Try to first nail down which market you want to concentrate and start following and research about it.