Geometry help please?

Visualize it this way: draw a kite. Now cut it along both diagonals so that you have 4 pieces. You will have 4 right triangles which, when flipped over and pushed together, make one rectangle 40' by 70'. That's 2800 sq. feet of area. 400 square feet go into 2800 sq ft. 7 times. That's 7 gals of sealant every 6 months, or 14 gal. per year,. 3 years x 14 gal = 42 gals.