Answer: 82
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Explanation:
Let A be the tens digit and B be the units digit
We can write the number as AB. So if A = 9 and B = 2 for instance, then the number is AB = 92. Keep in mind I'm not multiplying A and B here.
"The unit digit is a prime" means B could be any of these values {2,3,5,7}. We only list the single digit primes. The value 1 is not prime.
Now multiply each of those items by 4
4*2 = 8
4*3 = 12
4*5 = 20
4*7 = 28
Of those results, only 4*2 = 8 leads to a single digit answer.
This must mean that A = 8 and B = 2
Therefore the number is AB = 82.
