Exegesis:

tis … ex humōn ‘which of you?’ cf. on 11.5.

merimnōn ‘by worrying,’ ‘by being anxious.’

dunatai epi tēn hēlikian autou prostheinai pēchun ‘can add a cubit to his lifetime.’ For hēlikia cf. on 2.52. In view of v. 20 and of the meaning of pēchus (see below) the rendering ‘life,’ ‘lifetime’ is here preferable. For prostithēmi cf. on 3.20.

pēchus lit. ‘cubit,’ ‘ell,’ a measure of length (length of a forearm, about 18 inches = 46.2 centimetres) here used to indicate a short span of time.

Translation:

Which of you, or, ‘is there anyone among you who,’ ‘certainly no one among you.’

By being anxious, or, ‘by-means-of his worrying’ (Batak Toba), “however much he worries about it” (Phillips), sometimes better placed at the end of the sentence. Further shifts or expansions may be necessary, e.g. ‘even if you are anxious can you ever add…,’ ‘however much you worry how to make yourself live longer you cannot add…’ (cf. Tzeltal).

Can add a cubit to his span of life, or, ‘can cause his life to become one span longer’ (Sranan Tongo), ‘can live a day more/longer.’ A term for cubit exists in most languages (e.g. ‘an axe handle long’ in Dan); but in many of them it cannot be used as a measure of time in this context; hence, ‘short space of time,’ “a single hour” (An American Translation), ‘a day,’ but better not ‘an instant/second,’ since the ‘cubit’ is not the smallest measure of length. His span of life, or, ‘the length/duration of his life,’ “his years” (The Four Gospels – a New Translation), ‘his-days’ (Batak Toba). Brouwer and Bahasa Indonesia RC use ‘path-of-life,’ in the sense of ‘length of life,’ with which term ‘cubit’ (as measure of length) fits in nicely.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.