If the consumer price index decreases from 250 to 150 do prices decrease by 100?

No, if the consumer price index (CPI) decreases from 250 to 150, it does not mean that prices have decreased by 100. The CPI is a measure of the average change over time in the prices paid by consumers for a basket of goods and services. A decrease in the CPI indicates a relative decline in price levels, but the actual change in prices is not simply the difference in index values; it reflects a percentage change in the overall price level.