Let the amount of time spent on the bicycle be x minutes.
Thus, the amount of time spent on the stair climber will be (30 - x) minutes.
Since 300 calories were burnt, we have:
9x + 11.5(30 - x) = 300
9x + 345 - 11.5x = 300
11.5x - 9x = 345 - 300
2.5x = 45
x = 45 / 2.5
x = 18
30 - x = 30 - 18 = 12
Hence, 18 minutes were spent on the bicycle and 12 minutes were spent on the stair-climber.