Answer:
$0
Step-by-step explanation:
The IRS Section 121 allows married couples who have purchased and lived in a house during the last 2 years a $500,000 capital gains exclusion ($250,000 for a single person). In this case it doesn't even matter what is Bill and Brenda's basis for the house, since the net selling price is $420,000 (it is less than the exclusion).