Question

I have five 10-rupee notes, three 20-rupee notes, and two 50-rupee notes in my wallet. If three notes were taken out randomly and simultaneously, what is the probability that at least 90 rupees were taken out?

Accepted Answer

B. Total ways to choose 3 notes out of 10 $= \binom{10}{3} = 120$. To get sum $\ge 90$: - Two Rs 50 + One Rs 20: $\binom{2}{2} 	imes \binom{3}{1} = 3$ - Two Rs 50 + One Rs 10: $\binom{2}{2} 	imes \binom{5}{1} = 5$ - One Rs 50 + Two Rs 20: $\binom{2}{1} 	imes \binom{3}{2} = 6$ Total favorable outcomes = $3 + 5 + 6 = 14$. Probability = $14 / 120 = 7 / 60$.

Probability of Note Values

Explanation

Practise this under exam conditions