Lizzy: Yolanda leaves home for work at 7:00 AM, riding her bike at 20 miles per hour. 15 minutes after she leaves, her husband realizes that she forgot her lunch, and jumps in the car to bring it to her. If he drives at 40 miles per hour and follows the exact same route as Yolanda, how many minutes will it take him to catch her?.
Me: Hmmm, let me think. I think this is the detailed solution:
Let x be the number of minutes it takes Yolanda's husband to catch her. We know that Yolanda will spend a total of x + 15 minutes riding her bike since she left 15 minutes before her husband. The distance each person travels is equal to their travel speed times the number of minutes they spend traveling. That means Yolanda's distance is equal to 20 mph * (x + 15) and her husband's distance is equal to 40 mph * x. Yolanda's husband catches up to her when they've both traveled the same distance, which is when 20(x + 15) = 40x. We can simplify this equation by multiplying 20 through the parentheses to get 20x + 300 = 40x. Then we can subtract 20x from each side to get 300 = 20x. Finally, we divide both sides by 20 to find that x = 15. Final answer: 15.