"Try that one with 2/3 + 1/7"
That would have to be 2*7 + 3 over 7 * 3 = 17/21.
ahh..so, just for fun, you multiply the numerator of fraction x (x=in this case 2/3) times the denominator of fraction y (y=in this case 1/7) and then add the result to the denominator of x and place that sum over the product of the x denominator and the y denominator.
Very good...and simple to understand too :)