Changing the selection pool

The universal set is 50 volunteers. Set A contains those trained in first aid and B those trained in food handling. The region counts are A only 16, both 12, B only 14, and neither 8.

1. One volunteer is chosen uniformly from all 50. Find the probability that the volunteer is trained in first aid.[1]
2. Instead, one volunteer is chosen uniformly from those trained in food handling. Find the probability that this volunteer is also trained in first aid.[2]
3. Explain why the denominators in your two calculations are different.[1]