The language specification helps us out here. Section 7.19 of the C# 5 spec states:
A constant expression must be the null literal or a value with one of the following types: sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, object, string, or any enumeration type. Only the following constructs are permitted in constant expressions:
- ...
- Cast expressions, provided the target type is one of the types listed above.
...
Whenever an expression fulfills the requirements listed above, the expression is evaluated at compile-time. This is true even if the expression is a sub-expression of a larger expression that contains non-constant constructs.
You can also validate this by looking at the IL, which in this case has:
IL_000a: ldc.r4 69.
when it's loading the divisor for the division operation.