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

### Instruction:
Which Effic is the highest one that has an Avg/G smaller than 305.6, and a GP-GS of 13-13?

### Inputs:
table_name: table, columns: ['Name', 'GP-GS', 'Effic', 'Cmp-Att-Int', 'Avg/G']

### Response:
SELECT MAX Effic FROM table WHERE Avg/G < 305.6 AND GP-GS = 13-13