Question 21915
the profit is proportional to total income. we don't know the growers other
expenses he has to pay out. 
my equation for total income is:
I = income
c = crates
#d = days passed
I = (400 c x $60 / c) + (20 c /d x #d)($60/c - ($2/d)/c x #d)
I = 400 x 60 + (20d)(60 - 2d)
I = 24000 + 1200d -40d^2
I = 2400 + 120 d - 4d^2
I = 600 + 30d - d^2
for a max or min I want I' the dirivative to be zero
0 = 30 - 2d
2d = 30
d = 15
I'll test for 14, 15 and 16 days to see if 15 days really is a maximum

{{{I = 400 * 60 + (20 * 14)(60 - (2 * 14))}}}
I = 24,000 + 8,960
{{{I = 400 * 60 + (20 * 16)(60 - (2 * 16))}}} 
I = 24,000 + 8,960
{{{I = 400 * 60 + (20 * 15)(60 - (2 * 15))}}}
I = 24,000 + 9,000
15 is a maximum
if you have not had calculus, you can use trial and error to get same result