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'X-dice' printed from https://nrich.maths.org/
This problem involves conditional probability.
Consider two dice $A$ and $B$, where the largest number on $A$ is
$N$. Then $P(A< B)$ is
$$
P(A< B) = \sum^N_{m=1}P(A< B|A=m)P(A=m)
$$
Of course, in this expression $P(A=m)$ is zero if the integer $m$
is not present on the die.
With problems such as these, don't be afraid to start with a period
of experimentation: just choose any numbers to begin with and
explore their properties. The structure of the problem will soon
start to emerge.