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a = 2
Using again the relation of order between $b$, $c$ and $d$ I found $b < 6$. Because $b > a$, I have to analyse $b = 3$, $b = 4$ and $b = 5$.
b = 3
From the relation between $c$ and $d$, and substituting the values for $a$ and $b$ I obtained $c < 12$, it means it could be 4, 5, 6, 7, 8, 9, 10 or 11.
a | 2 | 2 | 2 | 2 | 2 | 2 |
b | 3 | 3 | 3 | 3 | 4 | 4 |
c | 7 | 8 | 9 | 10 | 5 | 6 |
d | 42 | 24 | 18 | 15 | 20 | 12 |