Earning $40 in a day is quite difficult .
It depends up on your experience and abilities . You can earn it by gambling or trading but both the methods are quite risky. So , be careful !!
Its not wise to suggest gambling because instead of earning money you would lose even more if you do decide to engage with it.I would rather risk on trading because this is somehow a good way on earning on long term but doesnt really guarantee you anytime to make profits because earning money on this field would really depend on your skills and knowledge.There are lots of ways on earning on this forum but talking $40 it would really be difficult.