Help on homework...
John Hop-Doan Nguyen (154adjhn@pic.ucla.edu)
Sat, 11 Nov 95 13:48:32 -0800
In one of the last homework problems in 3.10, there was question asking for the
standard deviation for the length of a wall being built. If the bottom row
took 50 10-inch bricks and there was a 1/2 inch mortar between each brick, the
standard devation for each brick was 1/16 while the S.D. for the mortar was
1/32 in. If I let X=length of a brick, and Y=width of mortar, then Z= 50X +
49Y = the length of the wall.
If I solve for the variance of Z I get:
Var(Z)= Var (50X+49Y)= Var(50X) + Var(49Y)= 50^2 Var(X)+49^2 Var(Y).
but since, (S.D.)^2=Var, Var(Z)= 50^2(1/16^2) + 49^2(1/32^2)
Then to get the S.D. of Z, I square rooted the Var(Z). But I don't get the
right answer. Is there something wrong with my logic...
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