What happens to the influenza virus every 20-40 years, and why? What are the recent example(s)?

The influenza virus is mutated every 20-40 years.

Explanation:

Large mutations occur every 20-40 years. This results in a new influenza virus to  which the human population has no protection. These large mutations are almost always followed by an influenza pandemic.

Hope this helps!

water at room temperature is flowing through a pipe with pressure taps located 10 ft apart along one section of the pipe, which are connected to a mercury (s=13.6) manometer.

0.88m hope i am correct : )