Let us first recall these two ways of interference:

Constructive
Interference:
If two waves both have positive amplitudes meet and with equal amplitude their
resulting amplitude is twice of either of individual waves.(your ex.. +10 cm
and +20 cm wave.
Destructive
Interference:
If two waves but one with positive amplitude meets another amplitude with
negative value then their resulting amplitude will be smaller than either of
individual waves.(your ex.. +10 cm and –20 cm wave)

And as you rightly said the overall displacement of the wave is
as easy with simple arithmetic:


 

  

  
Displacement
  of Pulse 1
  
  

  
Displacement
  of Pulse 2
  
  

  
=
  
  

  
Resulting
  Displacement
  
 
 

  

  
+1
  
  

  
+1
  
  

  
=
  
  

  
+2
  
 
 

  

  
-1
  
  

  
-1
  
  

  
=
  
  

  
-2
  
 
 

  

  
+1
  
  

  
-1
  
  

  
=
  
  

  
0
  
 
 

  

  
+1
  
  

  
-2
  
  

  
=
  
  

  
-1
  
 
 

  

  

  

  
Tip: you could also estimate the position between the
various marked points and sketching the wave.( green line in the
figure as shown)

Thanks
& Regards,
Vasantha
Sivaraj,

askIITians
faculty