Answer questions correctly to move the progress bar forward. Once the progress bar is complete, you've mastered the topic.
You and your friends are favored to win this year's Rough Mudder obstacle race. You come off the start line at a confident 10 kilometers per hour, but the obstacles take a toll on your stamina, each one slowing you down by 1 kilometer per hour. What is your speed (in kilometers per hour) after obstacle 5?