How many $3$ digit numbers can be formed using digits $1,2,3,4$ and $5$ without repetition such that the number is divisible by $6$
First Approach:
A number is divisible by $6$ if it is divisible by $2$ and $3$.
Now the possible combinations I found are
$(1,3,2)$
$(3,1,2)$
$(2,3,4)$
$(3,2,4)$
$(4,3,2)$
$(3,4,2)$
$(3,5,4)$
$(5,3,4)$
total $8$ ways.
Second Approach
Case1:
unit digit can be filled in only two ways $(2,4)$ for nos $(3,2,4)$
Tens digit can be filled in $2$ ways
Hundred digit can be filled in $1$ ways
the required number is 2*1*2=4 ways
Case2:
unit digit can be filled in only one ways $(2)$ for nos $(1,2,3)$
Tens digit can be filled in $1$ ways
Hundred digit can be filled in $2$ ways
the required number is 2*1*1=2 ways
Case3:
unit digit can be filled in only one ways $(4)$ for nos $(3,4,5)$
Tens digit can be filled in $1$ ways
Hundred digit can be filled in $2$ ways
the required number is 2*1*1=2 ways
So, Total ways=8
Is there still a better way to solve this problem?
Best Answer
A refinement of your first approach:
In numbers which end with $2$, the sum of the other two digits must be $4$ or $7$:
the sum of the other two digits is 4
the sum of the other two digits is 4
the sum of the other two digits is 7
the sum of the other two digits is 7
In numbers which end with $4$, the sum of the other two digits must be $5$ or $8$:
the sum of the other two digits is 5
the sum of the other two digits is 5
the sum of the other two digits is 8
the sum of the other two digits is 8