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This is pretty basic.

(4)(-5) = -20

because 4 is positive and 5 is negative.

(-4)(-5) = 4 x 5 = 20

because both 4 and 5 are negative.

Guess what? This works for division too.

20 � (-5) = -4

Of course -20 � 5 = -4

Finally, -20 � (-5) = 20 � 5 = 4