Assuming a half-life of 1599 years, how many years will be needed for the decay of 15/16 of a given amount of radium-226

Now we know that number of half-lives is 4.

The number of half-lives is a quotient of total time elapsed and length of half-life.
So, total time elapsed is a product of a length of half-life (1599 years) and the number of half-lives (4). Since 1599 × 4 = 6396, then a scientist will have to wait 6396 years for decay of 15/16 of a given amount of radium-226.

it is the most commonly accepted one

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.