Step one: Find area of triangular pool, 1/2BH=1/2(20x8)=80ft^2
Step two: Find the area of the rectangle, BH=30x15=450ft^2
Step 3: Subtract the area of the triangular pool from the area of the rectangular deck, 450-80=370ft^2
Sandy will need 370 square feet of wood to build her rectangular deck around her triangular pool.