- All the three digits are different.
- The number is divisible by 7.
- The number on reversing the digits is also divisible by 7.
There are four such numbers: 168, 861 and 259, 952. To find such numbers first list down all 3 digits divisible by 7. This will generate a large set. In this set remove those whose all digits are not different. Among the remaining ones look for the reverse digits pairings like the two pairs shown here.
(b) 4