A rule links 8 to 36 and links 12 to 78. Using the very same rule, 11 is linked to which number?

36 = 1 + 2 + 3 + … + 8 and 78 = 1 + 2 + 3 + … + 12, so the rule is 'add up all the whole numbers from 1 up to the given number', which is the same as $n\times (n+1)÷2$. For 11 that gives $11\times 12÷2=66$. The traps: 132 is $11\times 12$ with the halving forgotten, 121 is $11\times 11$, and 55 is the answer for 10, one step too early.