Read this and answer the question. If the question is unanswerable, say "unanswerable".

The City of Plymouth is divided into 20 wards, 17 of which elect three councillors and the other three electing two councillors, making up a total council of 57. Each year a third of the council is up for election for three consecutive years – there are no elections on the following "fourth" year, which is when County Council elections take place. The total electorate for Plymouth was 188,924 in April 2015. The local election of 7 May 2015 resulted in a political composition of 28 Labour councillors, 26 Conservative and 3 UKIP resulting in a Labour administration. Plymouth City Council is formally twinned with: Brest, France (1963), Gdynia, Poland (1976), Novorossiysk, Russia (1990) San Sebastián, Spain (1990) and Plymouth, United States (2001).

What fraction of the Plymouth council is elected each year?
a third