In US: 21 is for a year, so a month is 21/12;
In China, a month is 21/12x(1/3); a year is 21/12x(1/3)x12=7. OK?
Very simple: Tian's monthly salary is 1/3 of that in US, then his annual salary is just 1/3 of his annual salary in US, understand?
the "7 is from PRC for 3 mon" is absolutely a rumor.