Thread: Standard Deviation Problem

I would appreciate your help on the following problem:

A mason is contracted to build a patio retaining wall. Plans call for the base of the wall to be a row of 50 10-inch bricks, each separated by -inch-thick mortar. Suppose that the bricks used are randomly chosen from a population of bricks whose mean length is 10 inches and whose standard deviation is inch. Also, suppose that the mason, on the average, will make the mortar inch thick, but the actual dimension varies from brick to brick, the standard deviation of the thickness being inch. What is the standard deviation of the length of the first row of the wall?

Thank you for your assistance.

Originally Posted by hasanbalkan

I would appreciate your help on the following problem:

A mason is contracted to build a patio retaining wall. Plans call for the base of the wall to be a row of 50 10-inch bricks, each separated by -inch-thick mortar. Suppose that the bricks used are randomly chosen from a population of bricks whose mean length is 10 inches and whose standard deviation is inch. Also, suppose that the mason, on the average, will make the mortar inch thick, but the actual dimension varies from brick to brick, the standard deviation of the thickness being inch. What is the standard deviation of the length of the first row of the wall?

Thank you for your assistance.

The length of the wall is:



Where the 's are RVs corresponding to the length of the 50 bricks and the 's are RVs corresponding to the thickness of the mortar between consecutive bricks, and these are all independedntly distributed.

The rest is just routine.

CB