1st day of last period 1/31. unprotected sex early hours of 2/6. plan b taken morning 2/7. age 50 1/2. what is probability of pregnancy?

Pregnancy at age "50" is exceedingly rare, and ovulation is frequently delayed or absent at this age. To be certain about pregnancy, however, obtain a serum HCG test.

Pregnancy is highly unlikely under the circumstances you described. Do a home pregnancy test, if you were pregnant, it would be positive by now. To be sure, you could get a blood test for HCG level. Wish you good health!