1. Let there is a fraction  12/28. 

Now we choose a number so that we can divide both numerator and denominator.  

Here if we choose 4 then

12/28 =  3/7     (12 and 28 are divided by 4)

Now there is no number exist which can divide 3 and 7 both

So 3/7 is the lowest fraction of 12/28.

2. Again let  0.30

So 0.30 = 30/100 

Now we choose a number so that we can divide both numerator and denominator.  

Here if we choose 10 then

30/100 =  3/10     (30 and 100 are divided by 10)

Now there is no number exist which can divide 3 and 10 both

So 3/10 is the lowest fraction of the decimal number 0.30