Answer:
Part 1:
700/x + 700/(x - 30) = 20
700(x - 30) + 700x = 20(x² - 30x)
35(x - 30) + 35x = x² - 30x
35x - 1,050 + 35x = x² - 30x
70x - 1,050 = x² - 30x
x² - 100x + 1,050 = 0
Part 2:
x = (100 ± √(100² - 4(1)(1,050)))/2
= (100 ± √5,800)/2
= (100 ± 10√58)/2
= 50 ± 5√58
= 11.92, 88.08
Part 3:
By coach: 55.08 km/h
By car: 88.08 km/h