Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Clubs | Seasons at this level | Settlements | Season joined league | Position in 1959–1960 |
| --- | --- | --- | --- | --- |
| Benfica | 27 seasons | Lisbon | 1934–1935 | 1 |
| Sporting CP | 27 seasons | Lisbon | 1934–1935 | 2 |
| Belenenses | 27 seasons | Lisbon | 1934–1935 | 3 |
| Porto | 27 seasons | Porto | 1934–1935 | 4 |
| Académica de Coimbra | 26 seasons | Coimbra | 1949–1950 | 6 |
| Vitória de Guimarães | 17 seasons | Guimarães | 1958–1959 | 7 |
| Atlético CP | 15 seasons | Lisbon | 1959–1960 | 11 |
| Barreirense | 14 seasons | Barreiro | 1960–1961 | Segunda Divisão |
| Sporting de Braga | 13 seasons | Braga | 1957–1958 | 12 |
| Sporting da Covilhã | 12 seasons | Covilhã | 1958–1959 | 9 |
| Lusitano de Évora | 9 seasons | Évora | 1952–1953 | 10 |
| CUF Barreiro | 8 seasons | Barreiro | 1954–1955 | 5 |
| Salgueiros | 5 seasons | Porto | 1960–1961 | Segunda Divisão |
| Leixões | 4 seasons | Matosinhos | 1959–1960 | 8 |
Question:
What is the "season joined league" for the club that had 15 seasons at this level?
Answer:
1959–1960