Q. Let $A$ be the set of 4 digit number $a_{1}a_{2}a_{3}a_{4},\ where\ a_{1} < a_{2} < a_{3} < a_{4},$then $n(A)$ is equal to
A) 84
B) 126
C) 210
D) None of these
Explanation
(b)
Required number of ways $= \ ^{9}C_{4} = 126$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.