But here is it multiplicative. Reason and experimentation suggests the formula for the Nth time is:

delay wait = interval Seconds * (backoff rate) ^ (attempt - 1)

But we would expect for the 4 attempts to be

> 30 * 5 ** 0
30
> 30 * 5 ** 1
150
> 30 * 5 ** 2
750
> 30 * 5 ** 3
3750

See similar here: https://exponentialbackoffcalculator.com/