To add or subtract fractions, you need to put them over a common denominator. Convert 1/2 into a fraction over a denominator of 64 by multiplying it by 32/32 (32/32 = 1, so we aren't changing the value of the fraction):
(1/2)(32/32) - 1/64 = 32/64 - 1/64 = 31/64
