In a six-digit number, the sixth, that is, the rightmost, digit is the sum of the first three digits, the fifth digit is the sum of first two digits, the third digit is equal to the first digit, the second digit is twice the first digit and the fourth digit is the sum of fifth and sixth digits. Then, the largest possible value of the fourth digit is

Let the six-digit number be text{ABCDEF}. The given conditions are: F = A+B+C, E = A+B, C = A, B = 2A, and D = E+F.Thus, D = 2A+2B+C = 2A + 4A + A = 7A.The first digit A cannot be 0, as it is a six-digit number.Also, A cannot be 2, as D would become a two-digit number.Consequently, A must be 1, which makes D = 7.