Solving for price incease The following table lists the basket of goods in the 
  Consumer Price Index for the nation of Astro​ (Assume 2014 is the base​ year.) 


  Table 6.2  
             2014     2014  2015     2015  2016     2016
             quantity price quantity price quantity price
  Sunglasses  8        $9      10      $11   7       $10
  Toothpaste  15        3      18       4    15       4
  Ferrets     6         12     4        15   9        18



  Using the information in Table​ 6.2, the percent increase in prices over the two year period from 2014 to 2016 is approximately  


The correct answer is 31% but I kept on getting 35%. 
My solution was (CPI of 2016 - 100)/100
The Consumer price index of a country is defined as:

$$CPI_{t}^{t_0}={{P_{1t} Q_{1t_0}+P_{2t}Q_{2t_0}+P_{3t}Q_{3t_0}}\over{P_{1t_0}}Q_{1t_0}+P_{2t_0}Q_{2t_0}+P_{3t_0}Q_{3t_0}} \times 100$$

where $P$ is price, $Q$ is quantity, $t$ is reference to current year and  $t_0$ is reference to the base year.

So using this formula we get: 

$${{10\times8 +4 \times 15+18 \times 6}\over{9 \times 8+3 \times 15 +12 \times 6}} \times 100 \approx 131.2 $$

Now finding the inflation rate we can see (CPI of the base year is always going to be $100$)

$${{131.2-100}\over{100}} \times 100 \% \approx 31\%$$

Hope this is helpful!