2 Answers

I just answered this the other day.  What's going on here??
chelleanne is correct.   The denominator (bottom) numbers must be the same.   Picture a pizza cut into 6 pieces and another pizza cut into 8 pieces.  If you had 2 of one and 5 of the other, how much of a whole pizza would you have?

2/6    +    5/8    =   N
2/6 is the same as 1/3.........   1/3  +    5/8    =   N
You need to find the "lowest common multiple"  for 3 and 8.  That is the smallest number that 3 and 8 are both factors.  That means 3 and 8 are numbers that can be multiplied with another number to reach the same product    A product is the answer in multiplication.  

Using the larger denominator (8), start multiplying until you come to a product that also has 3 as a factor.   8 x 1 = 8  (nope)     8 x 2 = 16  (nope   3 x 5 is 15.   3 x 6 is 18)    8 x 3 = 24  (OK! That works for obvious reasons)

NOW.....if we are changing the denominators to 24, we must make equal adjustments to the numerators......    You have to multiply 1/3  by 8    8/24.     You multiply 5/8 by 3     15/24

NOW you can add the numerators.  As stated previously, the denominator is not added.
8/24   +    15/24     =    23/24        And there's your answer.  To make your pizza look like this:
take the pizza cut into 6 pieces and cut each remaining piece into 4 pieces.  You have two pieces left, each cut into 4 piece = 8 pieces.
take the pizza cut into 8 slices and cut each of the remaining 5 pieces into 3.  You'll have 15 pieces.    15 pieces of 1 and 8 of the other is 23 of 24 total pieces that would equal 1 pizza.   

The denominator MUST be the same number. You do not add the denominator. You ONLY add the NUMERATOR. 

Ex.1    

 2/5 + 1/5 = 3/5

7/8 + 3/4 = 

         3/4 * 2/2 = 6/8

So, 7/8 + 6/8 = 13/8

13/8 = 1 5/8

Ex. 2

2 3/4 + 3 1/3 =

   3/4*3/3 = 9/12

   1/3*4/4 = 4/12

So, 2 9/12 + 3 4/12 = 5 13/12 

                                           13/12 = 1 1/12

5 + 1 1/12 = 6 1/12