The sum of the squares of the digits of a certain two-digit positive integer is 41. The integer is six times the sum of its digits. Find the integer.
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The only way to write 41 as the sum of two squares is 4 1 = 4 2 + 5 2 = 5 2 + 4 2 .
So the number either is 45 or 54.
We can simply check if these satisfy the second condition and in fact, 5 4 = 6 ⋅ ( 5 + 4 ) while 4 5 = 6 ⋅ ( 4 + 5 ) .
So the answer is 5 4 .
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Let the 2 digit number be 10t + u, where t is the tens digit, u is the units digit. We then have the following 2 equations: (1) t^2 + u^2 = 41, which tells us that t,u are 4 and 5 in some order. (2) 10t + u = 6(t + u) = 6t + 6u. Then 4t = 5u, and it is clear that t = 5, u = 4; so 10t + u = 54.