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The social security income calculation is based on the highest 35 years of a beneficiary's all working years. Therefore, unless a person's new working years of that 20 years replaced the highest 35 years previously in a significant way, the social security income payment amount would have been about the same. The below citation and website link are for your further reference.
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Fiona Chen, MPA, Ph.D., CPA, ABV, CFF, CITP
"Then Social Security calculates your average indexed monthly earnings during the 35 years in which you earned the most. We apply a formula to these earnings and arrive at your basic benefit, or “primary insurance amount.”