Average question GRE?

The average daily temperature from 9th to 16th January(both inclusive) was 38.6 C and that from the 10th to 17th January(inclusive) was 39.2 C. what was the temperature on 17th January? I am able to find total temperature as 308.8 C from from 9th to 16th January and 313.6 C from the 10th to 17th January but stuck!