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Question

The sum of the digits in the unit's place of all numbers formed with the help of 3,4,5,6 taken all at a times is

A
18
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B
432
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C
108
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D
144
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Solution

The correct option is C 108
Let 3 be the unit place number, then there are 3 choices for 10th place number, as the 10th place digit can be chose from 4, 5, 6. Similarly there are 2 choices form 100th place digit and 1 choice for 1000th place digit.
Hence there are 321=6 numbers ending with 3. These 6 number will add 6×3=18 to the sum of the digits in the unit's place.
Similarly, there will be 6 numbers each ending with 4, 5 and 6 and they will add 46=24;
56=30 and 66=36 to the sum.
Hence, the
sum of the digits in the unit's place of all numbers formed with the help of 3,4,5,6 taken all at a times is 18+24+30+36=108.

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