How Long Will It Take For 25 Of The C 14 Atoms In A Sample Of C 14 To Decay

How long will it take for 25% of the C-14 atoms in a sample of C-14 to decay?     

for this one the answer is t=2400

If a sample of C-14 initially contains 1.6 mmol of C-14, how many millimoles will be left after 2225 years?     

I don’t understand how to solve this.

 

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