What percentage of group B Rh positive donors would be expected to be crossmatch-compatible with Mr. FS?

According to probability theory, if p(A) is the probability that A will occur, and p(B) is the probability the B will occur, the probability of p(A, B) (i.e., that both A and B will occur) is calculated as:

p(A, B) = p(A) x p(B)

Put another way, the probability of two events happening together is derived by multiplying each probability.

In a Caucasian population the relevant antigen frequencies are as follows:

Antigen % Positive % Negative
C 67 33 Jka 77 23

The calculation for the percentage expected to be both C- and Jk(a-) is as follows:
0.33 X 0.23 = 0.076 or 7.6%