To convert the mixed number 3 1/8 into a decimal, we start by converting the fraction part (1/8) into a decimal. We know that 1 divided by 8 equals 0.125. Now, we add this decimal result to the whole number part (3).
So, 3 + 0.125 equals 3.125. Therefore, 3 1/8 as a decimal is 3.125.