For the fraction 28/10,
times 10 at 28 and 10,(to get the same denominator"100")
28×10=280
10×10=100
Thus,
28/10 become 280/100
We can plus the number above the denominator"100",which is 39 and 280.It is because they have achieved a same denominator.
39/100+280/100
=(39+280)/100
=319/100