Question 1084467
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Based on an identity (x-1)*(y-1) = xy - (x+y) + 1, substitute x= 50, y= 20 and do this:


1.  Calculate 50*20 = 1000 (mentally).


2.  Calculate 50 + 20 = 70 (mentally).


3.  Subtract 70 from 1000 to get 930 (mentally).


4.  Finally, add 1 to 930 to get 931 (mentally).
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Done.