Answer: 900.
- A 900
- B 810
- C 9000
- D 90
Correct answer: A. 900
Explanation: Form abcba: 'a' determines 1st & 5th, 'b' 2nd & 4th, 'c' middle. a ∈ {1–9}: 9 choices; b ∈ {0–9}: 10; c ∈ {0–9}: 10. Total = 9×10×10 = 900.
Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Concept context
Counting, arrangements, and selections