This question demonstrates what can happen if you use rounded numerical results too soon. Always keep enough digits in intermediate calculations to arrive at a final answer with sufficient precision. Consider the following problem.

Suppose you want to buy $100$ pens. You can buy $100$ pens at a $10\%$ discount at one store where the price is $$5\mathrm{.}99$per pen. And you can also buy $100$ pens at a $15\%$ discount at another store where the price is $$6\mathrm{.}41$per pen.