1800 ft² = 3 pounds
[divide by 3 through]
1800 ÷ 3 = 3 ÷ 3
600ft² = 1 pound
[8400 ÷ 600 = 14; so multiply by 14 through]
600 x 14 = 1 x 14
8400ft² = 14 pounds
we need 14 pounds to cover 8400 square feet.
[Find number of 4 pounds bags needed]
14 ÷ 4 = 3.5
So we need to use 4 bags of 14 pounds of lawn seeds.