Pooling in the Chi-squared test?

Hi, please take a look at this exam paper:

http://mei.org.uk/files/papers/s307ja_k39h9.pdf

Please read question 4(a) on page 4 of the pdf

Now scroll down to page 11 of the pdf to see the mark scheme answer.

What I don't understand is why they've pooled the first two groups.

Also, how do you determine how many degrees of freedom are lost? In other words, when you're looking at the table of values for the Chi-squared distribution, how do you know what row to read off? This seems to require knowledge of how many degrees of freedom are lost, can someone remind me of to determine this?