ab . ac . 7 = abbc
ab . ac . 7=100 . ab + bc
ab . ac . 7 - 100 . ab = bc
ab . ( ac . 7 - 100 ) = bc
=> ac . 7 - 100 < 10
ac . 7 < 110
ac < 16
=> a = 1
=>ac . 7 - 100 = 1c . 7 - 100 = c . 7 + 70 - 100 = c . 7 - 30 < 10
=> c . 7 < 40
=> c < 6 và c . 7 - 30 > 0
=> c . 7 > 30
=> c > 4
=> c = 5
=> 1c . 7 - 100 = 105 - 100 = 5
=>ab . 5 = bc
=> 1b . 5 = b5
=>50 + 5b =10b + 5
=>45 = 5b
=> b = 9
vậy a = 1 ; b = 9 ; c= 5