In a lot of 12 washing machines , there are 3 defective pieces . A person has ordered 4 washing machines . Find the probability that all the four are good .

My working :

X-B(4 , 0.75)

P(X=4) = 4C4(0.75)^4 (0.25)^0 = 81/256

But my answer is wrong . Where is my mistake again ? Thanks