# Reduction of SUBSET-SUM to SET-PARTITION [duplicate]

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There is a similar question that has been asked, but my question addresses particular detail of an answer.

I am trying to reduce SUBSET-SUM to SET-PARTITION. I found the following description:

SUBSET-SUM is defined as follows: Given a set X of integers and a target number t, find a subset Y from X such that the members of Y add up to exactly t. Let s be the sum of members of X. Feed X' = X U {s - 2t} into SET-PARTITION. Accept if and only if SET-PARTITION accepts.

What does "X' = X U {s - 2t}" (union?) and "Feed X' = X U {s - 2t} into SET-PARTITION" mean? How does this prove that SUBSET-SUM can be reduced to SET-PARTITION?