A single seed costs $.005 because .5 / 100 = <<.5/100=.005>>.005
It costs $.02 cents to grow an ear of corn because .005 x 4 = <<.005*4=.02>>.02
On each ear of corn he makes $.098 profit because .1 - .02 = .08
He sold 500 ears because 40 / .08 = <<40/.08=500>>500
#### 500