How do I add a variable amount for each amount of a variable

So basically, let's say for every 5 houses you buy, you lose 5 moneys every 7 seconds. How do I do this without programming something like this:

Every 7 Seconds:

Houses > 4

Lose 5 Money

Houses > 9

Lose 5 Money

Etc.

Is there a way to simplify this so I don't need to keep on manually placing a command that tests the amount of bought houses; example shown above. The command that I don't want to keep manually placing is underlined.

Instead, I want to be able to just place one command that keeps on checking the amount of houses I buy, so every 7 seconds it'll check to see if I bought another 5 houses which would then decrease my money more.

I'm really sorry if I made no sense, I'm really tired right now.

Every 7 seconds

-> Money - int(Houses/5)*5