I am reading a Paper by ROBERT I. JEWETT Spaces with an Abstract Convolution of Measures.
On page 51, he defines $K = \{ a,b,c\}$. The convolution operations are defined as \begin{align*} p_ap_a &= \frac 14p_e+\frac1{20}p_a+ \frac 7{10}p_b;\\ p_ap_b &= \frac 7{20}p_a+ \frac 3{10}p_b;\\ p_bp_b &= \frac14 p_e+\frac 3{10}p_a+ \frac9{20}p_b. \end{align*} My question is how did he get these values also the Haar Measure $m = p_e+4p_a+4p_b$?