I usually take my decimal numbers and multiply them up to make it an integer. Take this simple equation as an example:
15 * 3/5 = 9
Since 3/5=.6
I mulitiply .6 by 10 to get 6. And divide the whole thing by 10 afterwards.
(15*6)/10=9
I know this is kinda simple but with calculator in hand along with trial and error to check my math (I'm a little weak) I eventually get there.