A blank is usually multiplication because (1)1 means to multiply without any actual symbol, but on the other hand, addition, subtraction nor division will have a blank because their signs are different than multiplication which has many sorts of ways to multiply in any way that makes you comfortable.
5/20 can also be represented as 25/100 or 25%. The can be done by multiplying the top and bottom both by 5 and then dividing our numerator (25) by our denominator(100) to get the %.