OOT: Negative number multiplication

[Groundhog]

dear friends,

few weeks ago I had a question, acctually really simple question, but
until today nobody can give me answer. okay here is my question:

"as we know since Elementary School:
(+) x (+) = (+)
(+) x (-) = (-)
(-) x (+) = (-)
(-) x (-) = (+)

now when I change with numbers:
5 x 3 = 5 + 5 + 5 = 15 (everybody know this very well)
5 x (-3) = ....................... = -15 (everybody getting confused)
(-5) x (3) = (-5) + (-5) + (-5) = -15 (no problem with it)
(-5) x (-3) = ....................... = 15 (then, everybody get frustated)

could someone give answer about this 'elementary' question??

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