When adding a positive number with a negative number you will act as if you are subtracting the two numbers, then take the sign of the largest number (disregarding the signs).
In this case the largest number when you take the absolute value is 31 and its sign is negative. Your answer will have a negative sign.
-31 + 30 = -1
Hope this helped!
~Just a girl in love with Shawn Mendes
