The answer is 3.1%; 96.9%. The formula to find the remaining radioactive material is (1/2)ⁿᵘᵐᵇᵉʳ ᵒᶠ ʰᵃˡᶠ⁻ˡᶦᵛᵉˢ, meaning (1/2)⁵ is 0.031. Converting this to a percentage, 3.1% is the remaining radioactive material while 96.9% is the percentage of the stable isotopes.