Quote:
First step:
We should determine which 5 digits from given 6, would form the 5 digit number divisible by 3.
We have six digits: 0,1,2,3,4,5. Their sum=15.
For a number to be divisible by 3 the sum of the digits must be divisible by 3. As the sum of the six given numbers is 15 (divisible by 3) only 5 digits good to form our 5 digit number would be 15-0=(1,2,3,4,5) and 15-3=(0,1,2,4,5). Meaning that no other 5 from given six will total the number divisible by 3.
Second step:
We have two set of numbers:
1,2,3,4,5 and 0,1,2,4,5. How many 5 digit numbers can be formed using this two sets:
1,2,3,4,5 --> 5! as any combination of these digits would give us 5 digit number divisible by 3. 5!=120.
0,1,2,4,5 --> here we can not use 0 as the first digit, otherwise number won't be any more 5 digit and become 4 digit. So, total combinations 5!, minus combinations with 0 as the first digit (combination of 4) 4! --> 5!-4!=96
120+96=216
Answer: C.
Hi Bunuel,
I have an approach...but don;t understand where I am going wrong...Here it goes..
Step One: Find out all the five digit numbers without repetition from 0,1,2,3,4,5
Thus there are 5 spots to be filled from a group of 6..... ( something like this _ _ _ _ _)
The 1st spot can be filled in 5 ways (0 not included here so that it reamains a 5 digit number) and the 2nd spot in 5 ways ( 0 included here ) and the rest can be filled in
4 x 3 x 2 ways..therefore the total number amounts to - 5 x 5 x 4 x 3 x 2 = 5 x 5! = 600
Step Two: Divide this 600 by 3 to get the number of 5-digit numbers that are divisible by 3 . Therefore 600/3= 200.
This 2nd step I have done keeping in mind th approach that we use for sums like these -
The number of numbers that are divsible by 7 between 300 and 500 are ?
Ans: Here's what I do...get a multiple of 7 just greater than 300, thts 301 and a mulitple of 7 just less than 500..ths 497..
then 497-301 = 196..then 196/7 =28....the answer is 28 + 1 = 29( +1 for counting one of the left out multiples within the range ).,..
With the above method, what I am basically doing is finding out how many numbers lie in the range and then divide it by 7..and then add 1 to get the answer...
Similarly, when I am using this in the given question on 5 digits, why am I not able to get eh correct answer ?
What I understand is tht in both these questions we are trying to find out all those numbers that are divisible by a particular number...
Please advise..
RJ