Answer: 17/40
Step 1: Write down the number as a fraction of one: 
 
0.425 = 0.425/1 
 
Step 2: Multiply both top and bottom by 10 for every number after the decimal point: 
 
As we have 3 numbers after the decimal point, we multiply both numerator and denominator by 1000. So, 
 
0.425/1 = (0.425 x 1000)/(1 x 1000) = 425/1000. 
 
Step 3: Simplify (or reduce) the above fraction by dividing both numerator and denominator by the GCD (Greatest Common Divisor) between them. In this case, GCD(425,1000) = 25. So, 
 
(425÷25)/(1000÷25) = 17/40 when reduced to the simplest form.