6,502 francs per month (equivalent to US$6,597 per month). After rent, taxes and pension contributions, plus spending on goods and services, the average household has about 15% of its gross income left for savings. Though 61% of the population made less than the mean income, income inequality is relatively low with a Gini coefficient of 29.7, placing Switzerland among the top 20 countries. In 2015, the richest 1% owned 35% of the wealth. Wealth inequality increased through 2019. About 8.2% of the population live below the national poverty line, defined in Switzerland as earning less than CHF3,990 per month for a household of two adults and two children, and a further 15% are at risk of poverty. Single-parent families, those with no post-compulsory education and those out of work are among the most likely to live below the poverty line. Although work is considered a way out of poverty, some 4.3% are considered working poor. One in ten jobs in Switzerland is considered low-paid; roughly 12% of Swiss workers hold such jobs, many of them women and foreigners. Education and science Education in Switzerland is diverse, because the constitution of Switzerland delegates the operation for the school system to the cantons. Public and private schools are available, including many private international schools. Primary education The minimum age for primary school is about six years, but most cantons provide a free "children's school" starting at age four or five. Primary school continues until grade four, five or six, depending on the school. Traditionally, the first foreign language in school was one of the other Swiss languages, although, in 2000, English was elevated in a few cantons. At the end of primary school or at the beginning of secondary school, pupils are assigned according to their capacities into one of several sections (often three). The fastest learners are taught advanced classes to prepare for further studies and the matura, while other students receive an education adapted to their needs. Tertiary education Switzerland hosts 12 universities, ten of which are maintained at cantonal level and usually offer non-technical subjects. It ranked 87th on the 2019 Academic Ranking of World Universities. The largest is the University of Zurich with nearly 25,000 students. The Swiss Federal Institute of Technology Zurich (ETHZ) and the University of Zurich are listed 20th and 54th respectively, on the 2015 Academic Ranking of World Universities. The federal government sponsors two