What could be the largest possible value of A?
Q*D '=' C (there may be a carry over)
1*2 = 2 X (can't repeat digits)
1*3 = 3 X (can't repeat digits)
1*4 = 4 X (can't repeat digits)
1*6 = 6 X (can't repeat digits)
2*3 = 6 possible (so C can be 6)
2*4 = 8 X (no 8)
2*6 '='2 X (can't repeat digits)
3*4 '='2 X possible so C can be 2
3*6 '='8 X (no 8)
4*6 '='4 X (can't repeat digits)
So C can be 2 or 6.
When C=2, then Q=3, D=4 OR Q=4, D=3
When C=6, then Q=2, D=3 OR Q=3, D=2
Combine this with what Subotosh Khan posted above.