there are 4 of the 10-nf and 8 of the 100-nf capacitors which is a total of 12 items.
the probability of drawing a 10-nf is: 4 out of 12 =
the probability of drawing a 100-nf is: 8 out of 12 =
(a) "at least two" means: 2 or 3 or 4
Probability of 2 (10's) and 2 (100's): x
x
x
=
Probability of 3 (10's) and 1 (100's): x
x
x
=
Probability of 4 (10's) and 0 (100's): x
x
x
=
2 or 3 or 4: +
+
=
(b) without replacement
Probability of 2 (10's) and 2 (100's): x
x
x
=
Probability of 3 (10's) and 1 (100's): x
x
x
=
Probability of 4 (10's) and 0 (100's): x
x
x
=
2 or 3 or 4: +
+
=
=
=