Which era was the longest?

The longest geological era is the Precambrian, which lasted for about 4 billion years. Here are the steps to answer this question:

1. Understand the question: The question is asking about the longest era. In this context, it's referring to geological eras, which are large divisions of Earth's history.

2. Research: Look up information about the different geological eras. There are several resources available, such as textbooks, scientific articles, and reliable online sources.

3. Compare the lengths of the eras: The main geological eras are the Precambrian, Paleozoic, Mesozoic, and Cenozoic. The Precambrian is the longest, lasting for about 4 billion years. The Paleozoic lasted for about 290 million years, the Mesozoic for about 180 million years, and the Cenozoic (which we are currently in) has lasted for about 66 million years so far.

4. Answer the question: The longest era is the Precambrian.