If the graph has 1/3 length…
At least 20 minutes late: this would mean 20 minutes late or more.
20 or more minutes goes from 20 to 30. So that is a difference of 30 – 20 = 10. The distance of your rectangle is 10.
Length * width = (1/30) * 10 = 1/3 or 0.3333. The probability the friend is at least 20 minutes late is 0.3333.