The sum of normally distributed variables is also normal.

Work out the mean and standard deviation of and use those as the paramters for the distribution of the wall.

Hint: for independant variables

part (ii)

Once you have found the distribution of y

You want P(y>204) + P(y<196)

If you dont remember how to do that, you need to standardise the normal variable.

Ill do the first one:

Define:

Now:

The expression in brackets is jus a number, you can look it up in your standard normal table.

You should be able to do p(y<196) from that method or by general reasoning. If you are totally stuck you can read the spoiler below

Spoiler: