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.
This is something you should memorize. Specifically it is from the Empirical Rule (or 68-95-99.7 rule) which gives rough approximations of areas under the curve.