First you would find a common denominator, in this case it is 60. So you would multiply 11 and 12 by 5 and 7 and 20 by 3, getting this equation.
55/50 - 21/60
From this equation you would subtract 21 by 55. Getting this fraction as a final answer.
31/60.
Also if you want a decimal it would be 31 divided by 60. Then you would get your decimal answer being:
0.516666666666666666666666666666666666666666....