The "border number" of an n times n square is defined as the number of unit squares whose edges border the edges of a larger square. The border numbers of 1 times 1, 2 times 2, 3 times 3,4 times 4 and 5 times 5 squares are illustrated. What is the border number of a 20 unit by 20 unit square?

step-by-step explanation:

let pq = qr = rs = x, and xz = y and zu = z.

if the horizontal lines are parallel, then corresponding segments are in proportion:

substitute:

therefore 3 = y = z.

xu = y + z → xu = 3 + 3 = 6

-3(b-8)-5=9(b+2)+1

distribute

-3b+24-5=9b+18+1

add/ subtract

-3b+20=9b+19

20-19= 1

-3b+1=9b

9b+3b= 12b

1=12b

1 divided by 12 = 1/12

answer: 1/12 = b