Question 363454
P[{a,b,c}] = P[{a,c}] + P[{b,c}] - P[{c}].  Thus {{{1 = 5/8 + 7/8}}} - P[{c}]. Hence P[{c}] = 1/2.  Now P[{a,c}] = P[{a}] + P[{c}], so P[{a}] = 1/8.  Finally, P[{b}] = {{{1-1/2-1/8 = 3/8}}}.