In order to add fractions that are not alike, you will need to find the lowest common denominator. This is the least common multiple of the fractions. To add 2/3 plus 1/5, you will need to find the lowest common denominator. Here, it is best to think of all of the multiples of the denominators. If it helps, make a list. For 3, it is 3, 6, 9, 12, 15, etc. For five, the multiples are 5, 15, 20, 25, etc. The lowest number that they have in common is 15. This is your lowest common denominator. Next, you will need to multiply your numerator by the same number that you used to get 15 for the denominators. It should look like this: 10/15 +3/15.