Tritium (³H) Is Used in Glowing "EXIT" Signs Located Where

Multiple Choice

Tritium (³H) is used in glowing "EXIT" signs located where there is no electricity for lightbulbs.If the half-life of tritium is 12.26 years, what percentage of the original quantity of the isotope is left in the sign after 20.0 years?

A) 0.677%
B) 67.7%
C) 32.3%
D) 0.323%
E) 25.0%