Need help figuring out the value for the "missing mean" is shows an interaction between variables A and B, but no evidence of a main effect for either independent variable.

The following matrix presents the results of a two-factor experiment with two levels of factor A, two levels of factor B, and n = 10 subjects in each treatment condition. Each value in the matrix is the mean score for the subjects in that treatment condition.

Factor B

B1 B2

A1 10 50

A2 50

(The missing mean value is for A2 B2 in the lower righthand cell.)