Another Capital gains question

[Andy Young]

I am selling a multifamily I bought for 100,000. Over the course of owning it I have paid off $25,000 of what I owe on the mortgage.

I paid 25,000 and got a mortgage for the 75,000.

I am now selling it for 200,000.

I am getting really worried about the tax bill.

My questions are (and I appreciate any answers you can give)

1.  Do I have to pay tax on the original $25,000 money I invested?

2. Do I have to pay taxes on the $25,000 of the progress I have made on the principal?

3.  Do I have to pay tax on the money I pay back to the bank (the $50,000?)

4. I understand that I will have to pay back any depreciation I took on previous tax bills.  The formula for this is (building cost/27.5 * numbers of years owned?  OR is it this number multiplied by a percentage? (I am staring right at my tax return from 2017 and can not figure out where this number is!)

Thanks for any help!