You can't add fractions when the denominators are not the same - it's a rule in math you can't ignore!
Write out the two fractions you want to add. In this video, the equation to solve looks like this: 2/3 + 4/5 = ???
Start by making a common denominator. This is easy - just multiple the two denominators to eachother. In this equation, 3 x 5 = 15. 15 is your new denominator.
The next trick is called "cross multiplying." Multiple the numerator on one side with the denominator on the other side of the plus sign. When you multiple a numerator of 2 by the denominator of 5, you get 10. Write this new number down.
Repeat the process with the other numerator and denominator. For this equation, 4 multipled by 3 equals 12.
Take these two numbers and add them together to make your new numerator. In this equation, our new numerator is 22 (10+12=22). Put your new numerator over your new denominator for the final answer of 22/15!