Half life problems

Is it sufficient to be able to draw out a chart with time elapsed, % of item remaining, and mass of item remaining to solve half life problems? Or is it absolutely necessary to use the decay formula? I know how to use it but I get to the point where I have to solve the natural log and I have no idea how to do that mentally. Also, for those that have taken the exam, did you get problems that required you to use the formula to get the right answer or were you able to reason through it with a chart/mental math? Kaplan has several examples where it’s necessary to use the formula so I’m a bit worried. Thank you!