Final answer:
The primary health plan for Cash will be determined by the birthday rule, which assigns primary coverage to the parent whose birthday comes earlier in the calendar year, barring any specific rules set by the respective insurance companies.
Step-by-step explanation:
When both parents have health plans that cover their child, the determination of which plan is primary is often done through a standard known as the birthday rule. The birthday rule states that the health plan of the parent whose birthday (month and day, not year) comes first in the calendar year is the one that pays first. Therefore, if Shane's birthday is earlier in the year compared to Casey's birthday, then Shane's plan would be primary. If it's the other way around, then Casey's plan is primary. If both Shane and Casey have the same birthday, then other factors such as the length of time each parent has been covered by their plan come into play.
It's important to note that the birthday rule is a general guideline, and specific insurance companies or plans may have their own rules. Thus, it's essential to check with both insurance providers for the specifics of their coordination of benefits.