A Find the maximum heart rate first. The difference between 220 and 35 is 185 . The aerobic rate is 75% of this number, so multiply 10075​ by 185 to get 138.75. The fat-burning rate is 55% of 185 , or 10055​(185)=101.75. Now find the difference between these two heart rates: 138.75−101.75=37, which is (A).