Your first step is to bring your decimal point up to the top of the equation. The you divide the whole number into the decimal number. In this instance, 5 into 1.25. 5 goes into 1 0 times, so you put the zero in the first space. 5 goes into 12 twice, so you place a 2 up top, after the decimal. Then you subtract 10 from the 12 to leave you with a 2. This then gets added on to the 5, so you divide 2 into 25 giving you 5. So your answer is 0.15. You can then do this for any whole number and fraction.