You must either
change the fractions to equivalent fractions with a common denominator
change the factions to equivalent decimals.
In google type :
add fractions with different denominators
You wil find many, many sites.
can't do it. Fractions with different denominators are different animals. Think of the denominator as a unit. A half is not the same as a third, just as apples are not oranges.
So, in order to add them, you need to convert them into the same units. To add 2/3 + 1/2, you have to make them somehow the same kind of thing.
If you cut a pie into 6 pieces, two of those pieces make 1/3, and 3 of them make 1/2. So, to do the addition, change things up so that
2/3 + 1/2
= 4/6 + 3/6
Now everything is the same kind of object: sixths
Now you can add them up
4/6+3/6 = 7/6 = 1 1/6
So, learn about something called the Lowest Common Denominator. To add fractions with different denominators, convert them so they are all multiples of the LCD. Then they can be added.
2/7 + 3/5 + 1/3
= 30/105 + 63/105 + 35/105