You just convert the second fraction's denominator to equal the first's (and modify the numerator accordingly), then subtract the numerators, in this instance, you muliply the 5 by 4 and what you do the bottom, you must do to the top, so you multiply 3 by 4 and get 12. 18 - 12 = 6, giving you 6/20, which converted is 3/10.
