Compound Inequalities, Union - Definition & Examples

Andy and Selena are at a carnival when they come across a guessing game. A big jar is filled to the top with gumballs, and the goal is to guess how many gumballs are in the jar. $\\$ Andy takes one look and guesses that there are at least $400$, but no more than $500$. Selena's guess is some number between $600$ and $700$, inclusive. How would you express the union of their guesses in interval notation?