1. Each letter represents the same digit.
2. Once letters have been replaced with a digit, the "math" has to work.
3. Once a digit has been used for a letter, that digit can't be used for any other letter.
MY + HE = US
Possible Answer (there may be more than one right answer):
M=1, Y=2, H=7, E=8, U=9, S=0
12 + 78 = 90
Now you try this one:
PIG + PIG = OINK
There may be more than one right answer for this one as well. Give it a try and let me know what you come up with.