Simple problem about fractions

Just a Simple Application of Euler's Totient Function

$\phi(320) = 128$

$n$ must be less than 320 to make a proper fraction. The prime factors of 320 are 2 and 5, so we need to eliminate any $n$ that is a multiple of 2 or 5. 159 positive multiples of 2 are less than 320. There are 32 positive, odd multiples of 320, so subtracting 159 and 32 from the original 319 possible $n$ 's, we end up with 128 .