I have a question and I don't understand how the number is used as a new value and not a reference value. Can you explain?

"English majors spend an average of $120 per course on books. Humanities majors spend an average of $50 per course on books. What is the percent difference between the two amounts relative to the amount for humanities majors?" 120 (new value)-50(reference value)/50(reference value)*100%=%140"

Also, how do I know when to move the decimal over 2 spaces to the right? {the calculator came up with 1.4}

Thank you in advance.