Two integers are selected at random from integers 1 through 11. If the

1.	Two integers are selected at random from integers 1 through 11. If the sum is even, find the probability that both numbers are odd.
Answer» The integers from 1 through 11 are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Out of these, there are 5 even and 6 odd integers. Let A: Both numbers chosen are odd B: Sum of numbers is even at random S: Choosing 2 numbers from 11 numbers. Then, n(S) = ¹¹C₂ n(A) = ⁶C₂ (∴ There are 6 odd integers) As the sum of both chosen integers can be even if both are even or both are odd, so n(B) = ⁶C₂ + ⁵C₂ and n(A ∩ B) = ⁶C₂ ∴ P(A) = \(\frac{n(A)}{n(S)} = \frac {^6C_2}{^{11}C_2}= \frac{6\times 5}{11 \times 10} =\frac{3}{11}\) P(B) = \(\frac{n(B)}{n(S)} = \frac {^6C_2+^5C_2}{^{11}C_2}= \frac{6\times 5+ 5\times 4}{11 \times 10} =\frac{5}{11}\) \(P(A\,\cap\,B) = \frac{n(A\,\cap\,B)}{n(S)} =\frac{^6C_2}{^{11}C_2} =\frac{5}{11}\) ∴ P(A/B) = \(\frac {P(A\,\cap\,B)}{P(B)}=\frac{\frac{3}{11}}{\frac{5}{11}} =\frac{3}{5}\)

Two integers are selected at random from integers 1 through 11. If the sum is even, find the probability that both numbers are odd.

Answer»

The integers from 1 through 11 are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Out of these, there are 5 even and 6 odd integers.

Let A: Both numbers chosen are odd

B: Sum of numbers is even at random

S: Choosing 2 numbers from 11 numbers.

Then, n(S) = ¹¹C₂

n(A) = ⁶C₂ (∴ There are 6 odd integers)

As the sum of both chosen integers can be even if both are even or both are odd, so

n(B) = ⁶C₂ + ⁵C₂

and n(A ∩ B) = ⁶C₂

∴ P(A) = \(\frac{n(A)}{n(S)} = \frac {^6C_2}{^{11}C_2}= \frac{6\times 5}{11 \times 10} =\frac{3}{11}\)

P(B) = \(\frac{n(B)}{n(S)} = \frac {^6C_2+^5C_2}{^{11}C_2}= \frac{6\times 5+ 5\times 4}{11 \times 10} =\frac{5}{11}\)

\(P(A\,\cap\,B) = \frac{n(A\,\cap\,B)}{n(S)} =\frac{^6C_2}{^{11}C_2} =\frac{5}{11}\)