If the width of the deck is represented by w (in yards), the new area will be

120 = (8 + 2w)*(10 + 2w) = 80 + 36w + 4w^2

121 = 4w^2 + 36w + 81 (add 1 to make a perfect square)

11^2 = (2w + 9)^2 (rewrite as squares)

11 = 2w + 9 (square root)

2 = 2w (subtract 9)

1 = w (divide by 2)

The deck is

120 = (8 + 2w)*(10 + 2w) = 80 + 36w + 4w^2

121 = 4w^2 + 36w + 81 (add 1 to make a perfect square)

11^2 = (2w + 9)^2 (rewrite as squares)

11 = 2w + 9 (square root)

2 = 2w (subtract 9)

1 = w (divide by 2)

The deck is

**1 yard wide**on all sides of the pool.