If a, b, c, and d are replaced by four distinct digits from 1 to 9, inclusive, then what's the largest possible value of the difference a. b - c. d ?

8.6

Step-by-step explanation:

The units digits matter most, so we make a as large as possible (a=9) and make c as small as possible (c=1). Then we can use any of the digits from 2 through 8. We still want b as large as possible and d as small as possible so we set b=8 and d=2. This gives a maximum possible difference of 9.8 - 1.2 = 8.6.

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