Final answer:
An alloy best describes the mixture of 85% copper and 15% tin, which is known as bronze and is harder and more easily cast than pure copper.
Step-by-step explanation:
The word that best describes a mixture that is 85 percent copper and 15 percent tin is an alloy. An alloy is a mixture composed of two or more elements, where at least one of the elements is a metal. This specific mixture of copper and tin is known as bronze, which is commonly used because it is harder than copper and more easily cast.