The speed of the current is 27.5 mi/h.
Since the boat travels ds downstream in the same time it takes to travel us upstream, we can set the two speed equations equal to each other:
(s + c) * t = (s - c) * t
where t is the time taken to travel both downstream and upstream. We can simplify this equation to:
2s = ds + us
We know that ds = 3 mi and us = 2 mi, so we can substitute these values into the equation:
2s = 3 mi + 2 mi
2s = 5 mi
Now we can solve for s:
s = 5 mi / 2
s = 2.5 mi/h
We know that s = 30 mi/h, so we can set this equal to the equation for s and solve for c:
30 mi/h = 2.5 mi/h + c
c = 30 mi/h - 2.5 mi/h
c = 27.5 mi/h
Therefore, the speed of the current is 27.5 mi/h.