​(30)+(-50)-(20)

(30) + (-50) - (20)

= 30 - 50 - 20

= -40

30 + (-50) - 20 <-- the brackets are unnecessary

= 30 - 20 + (-50)

= 10 + (-50)

= 10 - 50

= -40

 

Same as hectictar's answer.