So distilling all this we have
5 earthlike planets
6 marslike planets
planets are distinct
earthlike planets use 2 units of colonists
marslike planets use 1 unit of colonists
There are 12 units of colonists to assign.
Probably easiest to work it looking at how many earthlike planets are colonized
There are only 6 marslike planets so there must be at least 3 earthlike planets colonized
There are only 5 earthlike planets so there can be at most 5 colonized
\(k ~Earth \Rightarrow 12-2k~Mars\\ N = \sum \limits_{k=3}^5 ~\left[\dbinom{5}{k}\right]\left[\dbinom{6}{12-2k}\right]\\ N=100\)
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