Tutorial Exercise Beginning with 20 grams of a radioactive element whose half-life is 35 years, the mass y (in grams) remaining after t years is given by 1 6/35 y = 20 t 20. How much of the initial mass remains after 160 years? Step 1 To find the mass that remains, substitute * for t into the following formula. y = 20(_) t/35 t > 0