To calculate the time constant (∆t) in this RL circuit, we can use the formula:

∆t = L / R

Given values:
L = 0.5 H
R = 15 ohms

Plugging in the values:

∆t = 0.5 / 15
∆t = 0.0333 seconds

So, the time constant ∆t is 0.0333 seconds.