A lawn fertilizer recipe uses ammonium sulphate, iron sulphate and sharp sand in the ratio 3 : 1 : 20. 100 grams of fertilizer are needed for each square meter of lawn.
a) How much ammonium sulfate is needed to make 100 g of fertilizer?
b) Dave's lawn is a rectangle, 4 m long and 9 m wide. How much sharp sand does he need to fertilize his lawn?