Decomposing is technically breaking a number apart, so technically you're taking your fraction and writing it as an expression. That line, in the fraction, is another way of saying that you have to divide. And you always divide the numerator (top number) by denominator (bottom number). Say your fraction is 3/4, you do. 3 divided by 4, in which case would be .75. But you do this to whatever fraction you have.