Question 12341
Let's say you want to add a/b and c/d

Then you can add as follows

(axc + bxd)/bxd

Let's try with 3/4 + 5/6

We re-write from above as 

(3x6 + 5x4)/4x6

=(18+20)/24

=38/24

=19/12

Hope this helps.