If I am in a shul in Flatbush where they use a katan as the tenth person of the minyan, what should I do? Walk out?
You should leave the minyan in a quiet and polite way, so as not to cause them to daven parts of the tefilah that they may not say without a real minyan. However, situations like these tend to be complex. It would be advisable to discuss with a Rabbi who is more familiar with the details of the situation.
All the best
Rabbi Shay Tahan