Explanation:
Factors of 21 are 1, 3, 7 and 21.
a + f + t cannot be 1 or 3 as it cannot be written as a sum of 3 distinct digits, so it is 7.
The smallest 3 remaining digits are 2, 5 and 6 which add to 13.
So there are no solutions.
Answer: 12
21 = 21 x1 3x7
=1x3 expand over3= 12
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