I need help ASAP please.

An accounts receivable account balance is $500,000 and the auditor determines a sample size of 25 would provide adequate assurance. The auditor plans to use PPS with systematic sample selection. The auditor notices that there are six customer accounts of at least $18,000 and would like the systematic selection technique to select all items that are at least $18,000, even if that means the sample size is slightly larger than 25. To achieve the auditor's objectives, the sampling interval should be