How can i be good at math in 2 weeks?

ok so my uncle wants me to do this little job thing and i just found out i have to calculate money and stuff so its basically stuff like someone will come up to me and hand me a $20 bill and they would say something like i want 2 tickets which are like $3.24 and they want change. i am not good at that crap and i need to learn how to do math stuff like that, btw i have 2 weeks and i have no idea what i am gonna do, plz help me out!!!