Final answer:
The auditor should treat the missing check in the same manner as the majority of the other 19 checks.
Step-by-step explanation:
The auditor should consider the reasons for the missing check and treat the missing check in the same manner as the majority of the other 19 checks. This means that if the majority of the other 19 checks were countersigned, the missing check should also be treated as countersigned. If the majority were not countersigned, the missing check should be treated as not countersigned.