An isotope of a radioactive element has a half-life of 2.5 years. How long will

An isotope of a radioactive element has a half-life of 2.5 years. How long will it take for the element to decay from 200 grams to 50 grams? Please show all work.
Balance both of the following chemical equations. Please show all work. 
a. Ni + O2® Ni2O3
b. F2 + HBr ® Br2 + HF

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