Question

Five different digits from the set of numbers {1, 2, 3, 4, 5, 6, 7} are written in random order. the probability that 5 digit number thus formed is divisible by 9, is

Answer 1

Heya User,

--> Maximum sum is :-> 28
--> Hence, sum of the five digits is either 9, 18, or 27 ... [ By divisibility ]

--> Now, No. of such five digit no.s is :-> 7C5 [ without repetition ]
-->                                                              = 21 such no.s

So, to get a sum of 9, 18 or 27 out of 21 such possible sums... 
--> The probability is 3/21 = 1/7 ...