12 31/100 is the mixed fraction because in a decimal all the numbers to the left are whole numbers and all the numbers to the right are numbers that are part of a whole. Just like in a mixed fraction there is a whole number and a fraction in a mixed number. Since 12 is in the left it is the whole number and since 31 is to the right and in the place value 31 is ending in the hundredths place so that's why the fraction is, 31/100. That's how you came to know that 12 31/100 is the fraction for 12.31.
