I could only solve in a regular way, which is:
E[X] = 1 + E[Y|X1 = a]
E[Y|X1 = a] = 1/4 * (1+ E[Y|X1 = a]) + 3/4 * (1 + E[Z|X1 = a, X2 = b])
E[Z|X1 = a, X2 = b] = 1/2 * (1 + E[Z|X1 = a, X2 = b]) + 1/2 * (1 + E[W|X1 = a, X2 = b, X3 = c])
E[W|X1 = a, X2 = b, X3 = c] = 3/4 * (1 + E[W|X1 =...