If she stopped at 8;05, and she started 20 minutes earlier, just imaging a clock at 8:05 and turn back time 20 minutes.
Five minutes would be 8:00, ten minutes is 7:55, fifteen minutes is 7:50, and twenty minutes is 7:45. This, she started at 7:45
Note: a² - b² = (a-b)(a+b)
9x² - 16y² = (3x)² - (4y)² = (3x - 4y)(3x + 4y)