A family travels 18 miles downriver and returns. 8 hours=round trip. Their rate in still H2O is 2X current.?

What is the rate of the current?

Let the current speed by x mi/hr.
Then the boat speed in still water is 2x mi/hr.

The speed upstream is 2x - x = x,
and downstream it is 2x + x = 3x.

If t hr. is the upstream time, the downstream time is 8 - t hr.

Equating the distances up and down:
xt = 3x(8 - t) ...(1)
t = 24 - 3t
t = 6 hr.

The current can have any rate except zero. Because the travel times are expressed as multiples of the current rate, it makes no difference and disappears from the equation (1).