Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_56 (effic INTEGER, avg_g VARCHAR, gp_gs VARCHAR)
### Question:
Which Effic is the highest one that has an Avg/G smaller than 305.6, and a GP-GS of 13-13?
### Answer:
SELECT MAX(effic) FROM table_name_56 WHERE avg_g < 305.6 AND gp_gs = "13-13"