The half-life of the isotope \[_{11}N{{a}^{24}}\]is 15 hrs. How much time does it take for \[\frac{7}{8}th\] of a sample of this isotope to decay [MP PET 1994]

• A.            75 hrs                                      
• B.            65 hrs
• C.            55 hrs                                      
• D.            45 hrs