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Prime Numbers

The units dig...

Question

The units digit of every prime number (other than $2$ and $5$ ) must be necessarily

A

1, 3

or

5

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B

1, 3, 7

or

9

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C

7

or

9

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D

1

or

7

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Solution

The correct option is D $1, 3, 7$ or $9$
All even numbers (other than $2$ ) are composite, so prime numbers cannot end with any of the digits $0, 2, 4, 6, 8$ .

Also, a number with unit digit $5$ will be divisible by $5.$

Therefore, the unit's digit of every prime number(other than $5$ ) must be necessarily $1, 3, 7$ or $9$ .

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(iv) (3 : 5) or (8 : 13)

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State, true or false :
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