Math for trading/investing conversion?

[oweaponx]

Greetings, all. I’m not exactly sure how to word this, so hopefully this makes sense. I’m trying to figure out how to set this up, maybe a ration?

Let’s say I’m looking at a REIT, or maybe a fund. However, instead of buying the full amount of the item, I’m looking at a part.

To start, let’s say, to try and keep the numbers easy, that the item starts at \$7,000. Later, it rises to \$7,500. Still later, it drops to \$7,250.

Now, let’s say I’m using \$20, as my start. How do I know what my equivalent will be, at \$7,500. Then, as it drops, what will my equivalent be, when I want to sell. I’m also looking to do similar, with a start of \$50, and \$100, as examples.

I hope this has made sense, and I appreciate help!

 

[HallsofIvy]

I am not sure what you mean by "20 dollars, as my start". Do you mean investing just 20 instead of the full 7000? If so your share is \frac{20}{7000}= \frac{2}{700}. So when it goes up to 7500 your share is \frac{2}{700}(7500) or 21.43. If it then goes down to 7250 your share is \frac{2}{700}(7250) or 20.71.