We know the area of a small square is 24 in².
Assuming the small rectangles are all equal in area, the total area of the window is equal to 6 x 24 = 144 in².
Now looking at the big window, the total area = width x length so:
144 = 18 x width
width = 144 ÷ 18 = 8 in
Both of these conditions must be true in order for the assumption that the binomial distribution is approximately normal. In other words, if 
and 
then we can use a normal distribution to get a good estimate of the binomial distribution. If either np or nq is smaller than 5, then a normal distribution wouldn't be a good model to use.
side note: q = 1-p is the complement of probability p
Here is my answer, hope to help you
