Write the number as a fraction of 1 (one):
0.641 = 0.641/1
Simultaneously multiply the numerator and denominator by 10 as many times as the digits after the comma (separation between the integer part and the fractional part of the number):
Since we have 3 numbers after the comma, multiply the numerator and denominator by 1000. So,
0.641/1 = (0.641 x 1,000)/(1 x 1,000) = 641/1,000.
Note that multiplying by 10 is the same as moving the decimal point one place to the right.
Once this fraction is irreducible, that is, it is in reduced form, we can no longer simplify it. The answer is the previous fraction.
Skandar