5/(x-5)=x/(x-5)-5/4. Multiply through by 4(x-5): 20=4x-5(x-5). 20=4x-5x+25=25-x, x=5. This is an extraneous solution because when x=5, the denominators become zero so division by zero occurs, and the result is meaningless.
If a bag contains 4 red marbles, 3 blue marbles, and 7 green marbles and a marble is randomly selected from the bag, what is the probability that it is blue? P=3/(3+4+7)=3/14 because each of 14 marbles is equally likely to be drawn.