500
An equation that makes sense for this scenario: Mr. Stiff loves singing Christmas carols. He sang for an unknown amount of time on Monday. On Tuesday, he had to grade a bunch of tests, so he was only able to sing for half as long as he did on Monday. He sang for 150 minutes altogether.
What is x - 0.5x = 150? (Where x equals the number of minutes that Mr. Stiff sang Christmas carols on Monday)