Answer: -5/2
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Step-by-step explanation:
0.4 = 4/10 = 2/5
The original slope as a fraction is 2/5
Flip the fraction to get 5/2, then flip the sign to get -5/2 which is the perpendicular slope
The orginal slope 2/5 and perpendicular slope -5/2 multiply to -1. This is true of any pair of perpendicular lines where neither line is vertical nor horizontal.
Rephrased another way: -5/2 is the negative reciprocal of 2/5.