Explanation:
Joe's climbing rate is not constant
3 × 2 = 6
8 × 1.5 = 12
12 × 1.1666666667 = 14
Time when climber reaches 30ft is unknown because climbing rate is not constant
Zas's climbing rate is 2.5
2 × 2.5 = 5
4 × 2.5 = 10
12 × 2.5 = 30
At 12 seconds, Zas reaches 30 feet.
Stephen's climbing rate is 4
2 × 4 = 8
4 × 4 = 16
6 × 4 = 24
30 ÷ 4 = 7.5
7.5 × 4 = 30
At 7.5 seconds, Stephen reaches 30 feet.