The least common denominator of 10 and 15 is 30. It's easiest to use trial and error to find the LCD. In this case, just multiply both numbers by 2 and see if that works. Since 15 doesn't go into 20, I tried multiplying 15 by 2 instead, and got 30, which is divisible by 2. If that doesn't work, move on to multiplying them by 3, then 4 four, and so on. However, in this case, it did work, so, since you multiplied 15 by two to get 30, multiply 7 by two to get the fraction 14/30. Then, since you multiplied 10 by 3 to get 30, multiply 2 by 3 to get 6/30. Then, add the two together to get 20/30 and simplify the fraction to 2/3.
2/10 + 7/15 Given.
6/30 + 14/30 LCD.
20/30 Add.
2/3 Simplify.