Distance travelled by a particle is proportional to time when the particle is moving with a constant velocity. This is because velocity is defined as the rate of change of displacement with respect to time, and if velocity is constant, then displacement will be directly proportional to time.

Mathematically, we can express this relationship as:

```

d = vt

```

where:

* d is the distance travelled

* v is the velocity

* t is the time

This equation shows that the distance travelled (d) is equal to the product of velocity (v) and time (t). If velocity is constant, then d will be directly proportional to t.