Answer
a. $0.255$
b. $0.509$
c. $0.002$
Work Step by Step
Total outcomes of 4 out of 12: $_{12}C_4$=495
a. 0 defective resistors
Choose 4 out of 9 good ones, $_9C_4$=126, $p=126/495=0.255$
b. 1 defective resistor
Choose 3 out of 9 and 1 out of 3, $_9C_3$*$_3C_1$=252, $p=252/495=0.509$
c. 3 defective resistors
Only 1 choice for the resistors, $p=1/495=0.002$
