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