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Answer:
The CPI increased 3.42% in Month 2.
Explanation:
In a spreadsheet, the weighted sum is easily computed using the SUMPRODUCT( ) function. It multiplies the corresponding values of two arrays, then returns the sum of those products. If one of the values is a weight, then the result is a weighted sum.
Your CPI is computed as the weighted sum of the prices of the individual products. The percentage change is computed in the usual way:
pct change = ((new value)/(old value) -1) × 100%
Of course, the spreadsheet can format a fraction as a percentage, so the computation of the change can stop after 1 is subtracted from the ratio.