Answer:
In order to add or subtract fractions, the denominator (bottom number) has to be the same. In order to make it the same, you find the LCM and multiply the fraction by whatever is necessary to make the denominator the LCM. FOR EXAMPLE:
1/3 + 2/5
The LCM of 3 and 5 is 15.
To make the 3 in 1/3 15, you multiply the whole fraction by 5 over 5 (it simplifies to 1 so you aren't really changing the fraction by multiplying it by 1).
1/3 * 5/5 is 5/15
You multiply 2/5 times 3 over 3 using the same principle
2/5 * 3/3 is 6/15
NOW you can add 5/15 and 6/15 to get 11/15.