Explanation:
what do you need ? the equation itself is true, if you put the same number in both places above 21.
any number is possible there, it will not change the equation.
so, again, I am not sure, what you need here.
let's write it with a variable to make my statements clearer :
9 + 1 x/21 = 10 x/21
that is true, because we can combine the whole numbers first giving us 9 + 1 = 10.
and then we add the fraction again (x/21).
again, you can put any number instead of x, and it will always be true.
example
9 + 1 4/21 = 10 4/21