Nope, they are not interchangeable at all :) 안녕하세요 is a polite form of saying hello, you may use it when you meet your elders, or formal occasions. 감사합니다 is also a polite form of saying thank you. The usage is similar but those two are totally different.

간단하다 is simple 쉽다 is easy I think they can be interchangeable.