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

### Instruction:
What is the casting temperature for the alloy with hardness 21?

### Inputs:
table_name: table, columns: ['Usage', 'Sn/Sb (%)', 'Liquid at (°C)', 'Casting at (°C)', 'Remelting at (°C)', 'Hardness']

### Response:
SELECT Casting at (°C) FROM table WHERE Hardness = 21