Answer:
The area of the bulletin board is 30 square foot.
Step-by-step explanation:
A bulletin board can be covered completely by 30 square pieces of paper without any gaps or overlaps.
If each piece of paper has side length of 1 foot so the area of each paper is 1 ft x 1 ft = 1 square foot.
There are 30 pieces of paper needed to completely cover the bulletin board, so 30 pcs of paper with an area of 1 square foot is equal to 30 square feet. The area of the bulletin board is 30 square foot.
Mean = 60 / 4 = 15
13 - 15 = -2
17 - 15 = 2
9 - 15 = -6
21 - 15 = 6
Total of squares = 4 + 4 + 36 + 36 = 80
because this is a sample we divide this by n - 1 ( = 3)
Standard deviation = sqrt (80/3) = 5.16 answer
The given expression is

The exponent of x is decreasing and y in increasing .
That is ,

Where a,b,c,d,e,f,g,h,i are th constants
So the correct options are b and d .