The denominator always has to be the same
Example: 1/3 plus 2/4
we know that the LCM (least common multiple) is 12 so you do 1 times 4 and 3 times for because what you do to the bottom, you have to do to the top and for 2/4 we multiply 2 times 3 and 4 times 3 that will leave us with 4/12 plus 6/12 and then you get 10/12 then you reduce that to 5/6. I hope this helps if it doesn't go to Khan Academy its a website and its on Youtube. Good Luck!
