The least common denominator of two fractions is 20. if you subtract the two denominations, you get 1. what are the denominators?

Steps
find the Least Common Multiple of the denominators (which is called the Least Common Denominator).

Change each fraction (using equivalent fractions) to make their denominators the same as the least common denominator.

Then add (or subtract) the fractions, as we wish!
your denominators stay the same so its 20

31/6= 19/6 19/6 *1/7=19/42