X is Y = X is less than Y - X is less than Y

from ABC180

• Number of cases where X is Y or less - Number of cases where X is Y-1 or less

• $c(X=Y)=c(X\le Y)-c(X\le Y-1)$

  • Equals less than subtracts less than

• count(X=Y)=count(X⇐Y)-count(X⇐Y-1)

• enumeration

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