(abc cba) mod(7) = 0
=> (1 × a + 3 × b + 2 × c + 6 × c + 4 × b + 5 × a)
mod (7) = 0
(6a + 7b + 8c) mod (7) = 0
(7(a + b + c) + (c – a)) mod (7) = 0
(c – a) mod (7) = 0
=> c – a = 0 or c – a = 7 or c – a = –7
Numbers = 5 + 5 + 5 = 15
=> Required 6-digit numbers = 45 + 10 + 15 = 70