Question 1111392
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Algebraically, if x is the units digit, then the number is 20+x and the sum of its digits is 2+x.  So<br>
{{{20+x = 3(2+x)}}}
{{{20+x = 6+3x}}}
{{{14 = 2x}}}
{{{x = 7}}}<br>
The units digit is 7; the number is 27.<br>
Using algebra to solve this problem is good practice with algebra; but the problem is easily solved with a little mental arithmetic.<br>
The number is 3 times the sum of its digits, so it is a multiple of 3.<br>
The only  multiples of 3 between 20 and 30 are 21, 24, and 27.<br>
Only 27 satisfies the condition that it is 3 times the sum of its digits.