The signed numbers are all part of the integers.
Positive numbers and negative numbers are the signed numbers.
When adding integers, for example -30+18=?, you look at the numbers, find out which number has a bigger absolute value (in this case it's -30), subtract like whole numbers (30-18=12) and then the sign of the number which has the bigger absolute value will be used.
When subtracting integers:
You change the subtraction sign to the addition sign and then you make the 5 to -5.
So, it will be -20+-5=-25