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:
| Year | 1851 | 1861 | 1871 | 1881 |
| --- | --- | --- | --- | --- |
| Year | 1851 | 1861 | 1871 | 1881 |
| Population | 1027 | 992 | 1017 | 813 |
| Year | 1961 | 1971 | 1981 | 1991 |
| Population | 634 | 683 | 687 | 764 |
| Census: 1801– 2001 2011 | Census: 1801– 2001 2011 | Census: 1801– 2001 2011 | Census: 1801– 2001 2011 | Census: 1801– 2001 2011 |
Question:
What's the 1851 when 1881 had 764?
Answer:
634.0