How to calculate interest on a mortgage loan?

How to calculate interest on a mortgage loan? - 1
Added (1). How to calculate 5% interest on a $40,000 mortgage loan

I'm sure the way I did it is incorrect. The condo I want is priced at $50,000 and after a 20% down payment, I would need a loan of $40,000.

I took $40,000, multiplied it by.05 and got $2,000. Then I divided $2,000 by 12 and got $166.66 a month on interest.

So, what is the correct way to calculate how much interest I would be paying each month on a $40,000 mortgage at 5%? Thanks.
Added (2). This would be a 30 year mortgage. I have already calculated everything else, I just need to know specifically how much the interest will be so I can add it to my monthly payment. Thanks.

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