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

### Instruction:
what is the [a] (mol/l) where [hpo 4 2− ]/[a] (%) is 0.612?

### Inputs:
table_name: table, columns: ['[A] (mol/L)', 'pH', '[H 3 PO 4 ]/[A] (%)', '[H 2 PO 4 − ]/[A] (%)', '[HPO 4 2− ]/[A] (%)', '[PO 4 3− ]/[A] (%)']

### Response:
SELECT COUNT [A] (mol/L) FROM table WHERE [HPO 4 2− ]/[A] (%) = 0.612