Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
In the sixth round, what is the sum of fixtures when there were more than 15 clubs involved?

### Inputs:
table_name: table, columns: ['Round', 'Date(s)', 'Clubs from the previous round', 'Clubs involved', 'Fixtures']

### Response:
SELECT SUM Fixtures FROM table WHERE Round = sixth round AND Clubs involved > 15