If the half-life of a radioactive substance is 86 hours, to the nearest hour, when will 85% of the substance have disappeared?

Question

Hello! I am having trouble answering and understanding this math problem. I would appreciate the help!

Answer 1

If 85% disappears, then 15% remains. Set

0.15 = 1 (1/2)^t/86,

and solve for t in hours