I once cut up an unripe papaya thinking it was ripe. I was about to throw it away when my roommate told me how to get it ripe without it spoiling.
She poured lemon juice over it, wrapped it up and kept it in the fridge for several days. Then, we washed it and ate it. It was still not perfectly ripe but much better than how it was when I first cut it.
It really takes experience to figure out if a papaya is ripe or not. I've finally learned but I've wasted a few and had to make smoothies out of a few in the process. By the way, putting a ripe tomato in the paper bag with the papaya works just as well as a ripe banana.