Question 1185430
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There is only one two-digit even positive integer number less than 90 with the sum of digits 15.


This number is 78.


<pre>
    The proof is OBVIOUS.

    The last digit must be even, and there is only one working possibility for it, which is "8".

    Hence, the tens digit is  15-8 = 7.
</pre>

So the grandfather's age is 78 years.


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Probably, the question's accurate form should be 


&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;how old is jack's and kylie's grandfather? &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(meaning one grandfather to two grandchild)

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;the digits of his age add to 15, his age is an even number, he is younger than 90.