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Let aX = the number of parts X produced by machine A;
aY = the number of parts Y produced by machine A;
bX = the number of parts X produced by machine B;
bY = the number of parts Y produced by machine B;
cX = the number of parts X produced by machine C;
cY = the number of parts Y produced by machine C.
Let P = aX + aY + bX + bY + cX + cY is the FULL NUMBER of all produced Parts.
Then the probability that the part X came from machine A is equal to , and we have
aX = (0.6*P)*0.4 = 0.24*P
bX = (0.3*P)*0.5 = 0.15*P
cX = (0.1*P)*0.7 = 0.07*P.
Therefore, the probability that the part X came from machine A = = = = 0.5217 = 52.17%.
Answer. The probability that the part X came from machine A = 0.5217 = 52.17%.