Question

A transformer has a primary voltage of 115 V and a secondary voltage of 24 V. If the number of turns in the primary is 345, how many turns are in the secondary?

Answer 1

Here's a fun and useful factoid:

               The ratio of the voltages on a transformer is the same
               as the ratio of the number of turns in each winding.

So the ratio of  (345 to the secondary turns)  is  (115V to 24V).

That's a proportion.

                 (115/24)  =  (345/x)

I'll bet you can take it and solve it from here.
Just cross-multiply in the proportion and etc. etc.