Correct Answer - C
Consider the following events :
E
1
→
E
He knows the answer,
E
2
→
E
He guesses the answer
A
→
A
He gets the correct answer.
We have,
P
(
E
1
)
=
90
100
=
9
10
,
P
(
E
2
)
=
1
10
P
,
P
(
A
/
E
1
)
=
1
,
P
(
A
/
E
2
)
=
1
4
P
∴
∴
Required probability
=
P
(
E
2
/
A
)
=
=
P
(
E
2
)
P
(
A
/
E
2
)
P
(
E
1
)
P
(
A
/
E
1
)
+
P
(
E
2
)
P
(
A
/
E
2
)
=
=
1
10
×
1
4
9
10
×
1
+
1
10
×
1
4
=
1
37
=
