Answer:
18
Step-by-step explanation:
A two-digit number (xy) will have the value 10x+y. The sum of its digits is x+y.
You want ...
10x +y = 2(x +y)
1 ≤ x ≤ 9
0 ≤ y ≤ 9
The equation can be simplified to ...
8x = y
The only single-digit values for x and y that satisfy the requirements are ...
x = 1, y = 8
The two-digit number of interest is 18.
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<em>Check</em>
Its sum of digits is 1+8 = 9. 2×9 = 18.
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<em>Comment on the relationship in the problem</em>
Many of us learned our multiplication tables for 9s by remembering that the sum of digits of 9n was 9, and that the leading digit of the product was n-1.
