The time taken for a wave to pass a point is 2.5 seconds. What is the frequency of the wave?

For this, we need to use the equation F = 1/T (where F = frequncy, and T = period).  The period of this wave is the time taken for one whole wave to pass i.e. from one wave crest to the next wave crest.  Here this is 2.5 seconds.

As such, the frequency = 1/2.5 = 0.4Hz (remember units)