Let's denote the total number of pages in the book as \( P \).
Chris read one-twelfth of the book in 6 days, meaning he read:
\[ \frac{P}{12} \]
in 6 days.
To find out how many pages he read each day during these 6 days, we calculate:
\[ \text{Pages read per day} = \frac{\frac{P}{12}}{6} = \frac{P}{72}. \]
Next, in the 20 days, he read the remaining pages after the first 312 pages. The remaining pages he read in 20 days is given by:
\[ \text{Remaining pages} = P - 312. \]
Since he read the same number of pages each day over 20 days, we can express this as:
\[ \text{Pages read per day over 20 days} = \frac{P - 312}{20}. \]
Now we know the number of pages read per day during the 20 days must be equal to the number of pages read per day during the first 6 days:
\[ \frac{P - 312}{20} = \frac{P}{72}. \]
Next, we solve this equation. We can cross-multiply:
\[ 72(P - 312) = 20P. \]
Expanding both sides gives:
\[ 72P - 22464 = 20P. \]
Now, we can rearrange this to isolate \( P \):
\[ 72P - 20P = 22464, \] \[ 52P = 22464. \]
Now, dividing both sides by 52:
\[ P = \frac{22464}{52} = 432. \]
Now that we have the total number of pages in the book \( P \), we can find how many pages Chris read in those 6 days:
\[ \text{Pages read in 6 days} = \frac{P}{12} = \frac{432}{12} = 36. \]
Therefore, the number of pages Chris read in those 6 days is:
\[ \boxed{36}. \]