Question

The half-life of a certain radioactive material is 78 hours. An initial amount of the material has a mass of 790 kg.. Write an exponential function that models the decay of this material. Find how much radioactive material remains after. 18 hours. Round your answer to the nearest thousandth

Answer 1

 half life i= 78 hours 
 amount after 78 hours is 395 kg: 
395 = 790e^(k*78) 

Dividing by 790 and taking natural log
ln (395/790) = (k*78) 
-0.6931 = 78k 
-0.00888 = k  

lets calculate  how much is left after 18 hours: 
Amount(18)
= 790e^(-0.00888*18) 
Amount = 673.301 kg
hope this helps