The tenths digit is the first one to the right of the decimal point, 3. The digit to its immediate right (6) is more than 4, so rounding is accomplished by adding 1 to the tenths digit (to make it 4), then dropping all digits to its right.
The rounded number is
12.4
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Alternatively, you can add half a tenth (.05) to the number, then drop all digits to the right of the tenths digit.
12.368 + .0.5 = 12.418
Dropping digits to the right of the tenths digit, you have 12.4.
