Am I divisible by double the sum of my digits?
JavaScript (ES6), 31 29 27 bytes

Takes input as a string. Returns zero for truthy and non-zero for falsy.

n=&gt;n%eval([...n+n].join`+`)


Commented

n =&gt; n % eval([...n + n].join`+`)
n =&gt;                                   // take input string n  -&gt; e.g. "80"
                  n + n                // double the input     -&gt; "8080"
              [...     ]               // split                -&gt; ["8", "0", "8", "0"]
                        .join`+`       // join with '+'        -&gt; "8+0+8+0"
         eval(                  )      // evaluate as JS       -&gt; 16
     n %                               // compute n % result   -&gt; 80 % 16 -&gt; 0


Test cases



let f =

n=&gt;n%eval([...n+n].join`+`)

console.log('[Truthy]');
console.log(f("80"))
console.log(f("100"))
console.log(f("60"))
console.log(f("18"))
console.log(f("12"))

console.log('[Falsy]');
console.log(f("4"))
console.log(f("8"))
console.log(f("16"))
console.log(f("21"))
console.log(f("78"))
console.log(f("110"))
console.log(f("111"))
console.log(f("390"))