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10d + 11c < 100 - a means that the max value for a=9, b=9, c=7, d=8
IMO, Odd man out is (C) 8581 since
- to get a 5 in the hundredth digit, b+b has to be 7+7 and get a carry over from tenth digit
- to give a carry over of 1, d+c = 18, but the maximum possible value for d+c = 15
The hundreds number of the sum could have several origins :
we know that it is at least equal to the units digit of 2.b (2.b could be above or below 10...)
you could have to add +1 if 10(d+c)+(c+a) >99 but we know from the stem that it is < 100 so only 2.b is contributing to the hundreds digit of the sum ; 2.b beeing even, the sole not compliant number is C