I'm not sure how the proper way to do it in school is, but for unfamiliar decimals like 0.108 you can turn it into fraction actually pretty easily. Since the decimals go out to the thousandths place, that means that your denominator is 1000 and the 3 digits are your numerator.
So 108/1000 is the fraction form. This can then by reduced to 27/250
When we learned how to convert fractions to decimals in school, we used long division, but I don't know what you guys use now.