How to calculate the CPI for expenditures?
Given:
2001 $645
2002 $545
Expenditures for what? In what year? What do the second "given" column of numbers represent?
I can't imagine CPI itself going down by that much (15.5%) from 2001 to 2002. It hardly ever goes down.
You must have left some information, as well as definitions, out of your question.