The answer is the number is 3 and the other number is 21. You can set the problem up as two equations. First, x = 1/7y as "a number (x) of 1/7 of another number (y)." Then the difference of the number is 18 is "y - x = 18" because y is the larger of the two numbers from the first equation. You could then substitute 1/7y for x in the second equation and solve for y, which is 21. You can then solve for x, the number, or 3.