Here they are just telling us that the apples make up a total of 3/7 percent of the total fruit in the box. So to find the total amount of fruit from this information we just simply take the n number of apples and divide that by the percent that it represents. So basically just this;
n=21 and (n)/(3/7)=total fruit
(21)/(3/7)=21*7/3=147/3=49 total fruit and 21apples make up (3/7)% of the total fruit in the box.
Hope this helped some. Any questions please feel free to ask. Thanks!