Answer:
222 years
Step-by-step explanation:
25% = 0.25
This radio active isotope have a half time if 111 years.
It means for every 111 years it decays to the half of the amount of it's substance present.
So in the first 111 years it will decay to it's half of the initial= 0.5
In another 111 year it will decay to another 0.5 of the present 0.5
That is, it will decay 0.5*0.5 = 0.25 if the original substance.
So I will take 111+111 = 222 years for the substance to decay to 0.25 or 25% of its original substance.
