What is the length of the major arc?

30pi is the exact

do pi*30 and round as needed for aproximate 

The circumference of a circle is given by pi*diameter.  The diameter here is 40 in, hence the circumference is 40pi in.  270° indicates that the arc is 3/4 of the circumference, so the length of the arc is (3/4)*40pi in or 30pi in.