Another approach to the 'highest averages' method, using a k9 dict:

    v: 10400 3400 6200
    s: 15

    quotients: ,/v%\.5+!s
    labels:    s/!s*#v              / list of 15x 0, 1, 2. arbitrary; could instead be party names
    top:       s#>labels!quotients  / k dicts can have duplicate keys; can sort items by key(^) or val(<>)
    seats:     `freq@!top           / count appearances of dict's keys

The ability to transform a dict's values while keeping the labels (keys) unchanged - sort, arithmetic, etc - is really powerful.

This answer is longer when minified, but it's faster in 2020.03.27:

    \t:100000 +/s>(#v)^<>,/v%/.5+!s           ⇒ 1494
    \t:100000 `freq@!s#>(s/!s*#v)!,/v%\.5+!s  ⇒ 691