1. Like 50 birds fly south each winter (50 is a lot of birds!)

2. At least half of all birds in the world fly south each winter (that's a lot more than 50!)

Mark and Susan don’t have a car. Their friend Anne does. Mark and Susan want to go to the beach. If mark and Susan go to the beach, then Anne must go to the beach.

M and S -> A

If Anne doesn’t go to the beach, then mark or Susan can’t go to the beach (because they don’t have a car).

/A -> /M or /S

Hope this helps someone

Gotta remember “only if” means “provided”