Hi another newbie question

hi,again,i was wondering if u could help me with sometjhing

i have a coin and an enemy they move left to right and wen they hit the border they go to the left side again but with other ._y so i was wondering i u could say me how to make a probabilite to the coin and the enemy appear like 40% that a coin appear and a 10% a enemy appear an wen i say appear i mean attach them dinamycally if i wasnt clear plz reply

:egg: ps: plz i need help fast