Combine the first two equations and laws of exponents to find the equation for the mass of sodium remaining after y years.

A half-life of 2.6 years means the mass is multiplied by 0.5 for every 2.6 years, so after y years, S = 800(0.5)^(y/2.6). Rewrite this as 800[(0.5)^(1/2.6)]^y; the yearly decay factor is about 0.766. Thus the equation is S = 800(0.766)^y, the choice stating that equation.

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