mean=50kg + 4(22) = 138 kg (right)
SD=[16 + 4(2.25)]=5
(c)
mean = 1380 $ ok
SD = 50 $
EDIT:
It's because:
mean(A+B+C+D+E)=
mean(A)+mean(B)+mean(C)+mean(D)+mean(E)
and
The weights of all the parts are independent of each other ==>
var(A+B+C+D+E)=
=var(A)+var(B)+var(C)+var(D)+var(E)
In your case
mean(B)=mean(C)=mean(D)=mean(E)
var(B)=var(C)=var(D)=var(E)
The weights of the wheels are independent of each other ==>
some could weigh less, some could weigh more.
In case of 4² it would mean that all 4 wheels have the same weight.
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