Minus Times Minus Equals Plus

Here’s an example I like of a negative number times another
negative number equaling a positive number:

I have been giving away five dollars each minute.
Currently, at time t = 0, I have zero dollars, but I am continuing
to give away five dollars each minute in the form of IOU’s.
Thus, the equation for the number of dollars I have at time t is
D = -5t, where D represents dollars.

Using that equation, calculate how many dollars I had four
minutes ago.

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