To simplify a fraction, you need only find a number that will fit into both, divide the numerator and denominator by that number, and keep going until you can't find such a number.
For 20/35, we'll use 5.
20 divided by 5 is 4
25 divided by 5 is 7
The reduced fraction is 4/7
And since there isn't any other number that will go into both other than 1, this is the simplest form.
So, the 20/35 in its simplest form is 4/7.