Final answer:
The AMT exemption for a married couple filing jointly with an AMTI of $128,900 is $111,700. This amount is deducted from their AMTI before applying the alternative minimum tax rate.
Step-by-step explanation:
The AMT exemption is an amount deducted from a taxpayer's alternative minimum taxable income. To determine the AMT exemption, we need to refer to the tax brackets for the current year. Unfortunately, the provided information does not mention the specific tax year. However, assuming the tax brackets for Year 2021, the AMT exemption for a married couple filing jointly with an AMTI of $128,900 is $111,700. This means that the couple would subtract $111,700 from their AMTI before applying the alternative minimum tax rate.