A school recorded how 120 students traveled to school and whether they arrived late.
| Travel method | Arrived late | Did not arrive late | Total |
|---|---|---|---|
| Bus | 12 | 48 | 60 |
| Car | 6 | 24 | 30 |
| Walk | 9 | 21 | 30 |
| Total | 27 | 93 | 120 |
If a student selected at random is known to have arrived late, which choice is the probability that the student walked to school?