There are two questions here. First Freddie: the interest on $600 is $30, so the interest on $100 is a sixth of the interest on $600, which is 30/6=$5. So the interest on each $100 is $5.
Now Emma: $1300 is 13 times $100, so the interest fees will be 13 times as much as they are for $100=13*5=$65.