How do you divide a decimal number by a whole number? It seems complicated but it really isn't when you get down to it. To divide a decimal number by a whole number you have to start off by dividing it as if it were any other number. Take for example 13.5 divided by three. If the number had been 135 instead of 13.5 the answer simply would've been 45 but as there is a decimal at work here, we will adjust the answer accordingly. Place the decimal in the middle to give the correct answer which is 4.5.