Answer:
The city's income base fell by $80 million
Explanation:
If during 2004 the city was able to raise $20 million with a 5% income tax, then the city's income base was:
($20 million / 5) x 100 = $400 million
If during 2005 the city was only able to raise $19.20 million with a 6% income tax, then the city's income base lowered to:
($19.20 million / 6%) x 100 = $320 million
This means that the city's income base was reduced by $80 million (= $400 million - $320 million)