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.