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.
#### 15