Probability of Note Values

Total ways to choose 3 notes out of 10 $= \binom{10}{3} = 120$. To get sum $\ge 90$:

• Two Rs 50 + One Rs 20: $\binom{2}{2} \times \binom{3}{1} = 3$
• Two Rs 50 + One Rs 10: $\binom{2}{2} \times \binom{5}{1} = 5$
• One Rs 50 + Two Rs 20: $\binom{2}{1} \times \binom{3}{2} = 6$ Total favorable outcomes = $3 + 5 + 6 = 14$. Probability = $14 / 120 = 7 / 60$.