The answer is: - (9/40).
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-2/21 * 27/40 = ?
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The "-2" cancels to "1" ; and the "40" cancels to "-20" ;
since: "40÷ -2 = -20" ; and since: "-2 ÷ -2 = 1".
The "21" cancels to "3" ; and the "27" cancels to "9" ;
Since: "21÷3 = 7" ; and since: "27÷3 = 9.
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So, we can rewrite the problem as follows:
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1/7 * 9/-20 = (1*9) / (7* -20) = 9 / -140 = - (9/40).
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