There's a rule of thumb that classes need to have an observed frequency of 5 or more to use the chi-squared test. One remedy is to pool. That's why the first two groups were pooled (because the first group has an observed frequency less than 5).What I don't understand is why they've pooled the first two groups.
The degrees of freedom is equal to the number of classes (after pooling) minus the number of parameters estimated from the population to calculate the expected frequencies (in this case two - the mean and the standard deviation of the normal distribution) minus 1.Also, how do you determine how many degrees of freedom are lost?
Therefore in this question, df = 6 - 2 - 1 = 3.