Route Summarization (Cisco)

Hi everyone,
i seem to get stuck when I get to the subject of Route summarization. Look at this: summarizing 172.16.16.0 through 172.16.31.0. From the book it summarizes this to 172.16.0.0/20. Using methods of summarization outlined (bits from the left which are common to all addresses) I got the summarized address as 172.16.16.0/20. Could someone please explain this?