The equivalent to the fraction 5/12 in decimal form is 0.4167. To be exact, the final number is a repeating six, which is written numerically as "6" with a horizontal line over it. If rounding is preferred, however, the last six is rounded up to a seven.
About.com demonstrates that to change a fraction into a decimal form, it is necessary to divide the top number (the numerator) by the bottom number (the denominator). In this case, the solution to five divided by 12 does not end but goes on infinitely repeating the last digit. In the case of 3/12, however, there are no repeating numbers, and the answer is simply 0.25.