The first step to determine the percentage of increase is to compute for the difference of consumer price index in value.
231.11 (consumer price index for 2012) – 211.4 (consumer price index for 2007) = 19.71 (increase)
To compute for the percentage of increase, you need to compare the increase to the consumer price index for 2007.
19.71/211.4 = 9.3% increase