Convert the fractions to the lowest common denominator. In this case the lowest common denominator is 40. Thus to convert each to 40ths, multiply 7/10 by 4/4 equalling 28/40 and multiply 3/4 by 10/10 equalling 30/40 and now you should be able to see that 7/10 is the smaller... Okay?
40 is actually a common denominator, but in this case, the lowest common denominator is 20. The above method is probably simpler than using the lowest common denominator though.