<s>[INST] The natural numbers $a, b, c$ and $d$ are such that:
\[ l(a,b) = 12 \]
\[ l(a,c) = 12 \]
\[ l(b,c) = 12 \]
\[ l(a,d) = 15 \]
\[ l(b,d) = 15 \]
\[ l(c,d) = 15 \]
where $l(x,y)$ denotes the LCM of $x$ and $y$. Given that $a$ is the smallest of these four numbers, find the value of $a$.[/INST]