Uncertainty calculations

1 adding or subtracting 

a=10+/-1
b=20+/-2
c=a+b=30+/-3
 d=b-a=10+/-3
2 multiplying or dividing

a=10+/-1
b=20+/-2
c=a*b=200+/-(1/10+2/20)*200=200+/-40
d=b/a=2+/-(1/10+2/20)*2=2+/-0.4