Plug in an actual number for k to make it easier. If we use 5, then there are 4 pieces left which makes 80% left. If you go through the answer choices and plug in 4 for k-1 and 5 for k, then [100 (k – 1)] / k or [100 (4)] / 5 = 400 / 5 = 80% (B). You can also just think of it as a fraction ((k-1) / k) multiplied by 100 to get the percentage.