What have you tried here?
To find a maximum with these types of problems often require you finding a derivative and solving for the zeros.
it costs you 10 euros per backpack to manufacture and distribute backpacks.if the backpacks sell at x euros each (where 15=or< x=or<100) the number sold is given by
where a and b are positive constants.what selling price will bring a maximum profit?
In this case is the cost of producing one unit so the cost of produceing units is , so you end up with and the profit on one unit is :
Now when you simplify the right hand side of the above you will end up with a quadratic expression in , and you have to find the maximum of this subject to the constraints on x.
(the last part can be done without calculus by completing the square)