3*16-2*4=48-8=40 Try this, its my only guess :(
The answer is

the key word here was at least, which means the farmer wants to make the equal or more than $500. Since the corn cost $2 and its selling per pound, depending on how many pound of corn (c) and the blueberries per pound (b) which are $4, leading to the first part of the inequality.
5 rational numbers between 0.5 and 0.6 can be any 5 numbers up to any number of decimal places that lies between 0.5 and 0.6. Or, it can also be 0.5124, 0.5676, 0.588, 0.59, 0.5789.S
Three plus the product of four times twelve