CFAI Mock 2009 AM

Q1, A, ii Shouldn’t the investable assets come down by 100,000 / 0.8 = 125,000, since they will need to withdraw enough that, after the 20% withdrawal tax, will yield 100,000 to pay off the mortgage?

the question says they need 100k for both mortgage and associated taxes. to reduce the investable assets by 125 k will be slicing off too much flesh