Please explain the underline sentences from radioactivity

In a sample of radioactive material, The percentage of the initial number of active nuclei will decay during one mean life = 63% and left 100-63 = 37%

nearly 37%

Roni, Just remember it in this way...in one mean time, 63% of the radioactive nuclei is gone and only 37% is left. The time taken by radioactive material to shed off 63% of origina...