Aswimming pool 20 m long and 10 m wide is surrounded by a deck of uniform width. the total area of the swimming pool and the deck is 704 m^2. find the width of the deck.

Let x = width of the deck.
Therefore total area of pool : 
(10+2x)m*(20+2x)m = 704 m^2
( 200 + 20x + 40x + 4x^2 ) m^2 = 704 m^2
(200 + 60x + 4x^2)   = 704
 4x^2 + 60x = 704-200
 4x^2 + 60x = 504
 4x^2 + 60x - 504 = 0
4(x^2 + 15x - 126) = 0
(x+21) * (x-6) = 0
Therefore x, the deck's width is 6m (it can't be -21 as width is measured as a positive value)
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If the diameter is 8 than the radius is 4 because 2r or 2*radius equals the diameter. 4*2=8. 8/2=4.