You have to find a common denominator for both fractions. I chose the denominator to be 10, so the 1/2 will become 5/10, and 1/5 will become 2/10.
Subtract 5/10 from 2/10. You will get -3/10.
Answer: -3/10
You can check your answer by solving 1/2-3/10=1/5 to see if it is true. (It is:)
