Which Prime Numbers?
Number Theory
Level
1
I have 2 prime numbers.
The last digit of the sum of these 2 prime numbers is 9.
What is the last digit of the product of these 2 prime numbers?
4
5
6
7
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1 solution
Prime numbers always end with odd digits.(Except for 2.)
But 9 cannot be the addition of 2 odd digits.
Therefore one prime number must be 2.
To sum up to 9 ,the last digit of the other prime number must be 7.
2*7 =14.
Therefore last digit of 14 is 4 .