Prepared in standard concentrations or percentages before use. Insulins, e.g., may be pure (composed of either short or long-acting varieties but not both), or they may be combined to take advantage of the differing times of onset and peak effect of two different agents. Premixed insulins are usually labeled with two numbers, separated by a slash (/). An insulin mixture that is half long-acting and half rapid-acting is labeled 50/50; one that is 70% long-acting and 30% rapid-acting is labeled 70/30.