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Well, there are many ways on how to solve this inequality.

Some might answer this using the method "guess and check' or what we call as "trial and error." There is another method without using the trial and error. It is like this:
   2a - 1 < 0
(since there is negative 1, to eliminate it, we add 1 to both sides)

2a - 1 + 1 < 0 + 1
(the result: negative 1 plus 1 is 0) (0 plus 1 is 1)

2a + 0 < 1
( 2a + 0 is still 2a)

2a < 1
(at this point, there is one number to eliminate, and that is 2 because it keeps on sticking to a) ( we divide 2 to both sides because if we divide 2a by 2, it results into a, thus, 2 will be eliminated in this process)

2a < 1 ( divide 2a and 1 by 2)

It becomes:

 \frac{2a}{2} <  \frac{1}{2}

So the answer is:
 a < 1/2
(which means, is lesser than 1/2) (which means a is any number as long as, it is lesser than 1/2)