See, they come in pairs: if $\gcd(i,75)=1$, then so is $\gcd(75-i,75)$. Obviously, sum of the numbers in each pair is 75; as for the number of pairs, it is half the number of numbers less than 75 and coprime to it, which is given by the totient function.
See, they come in pairs: if $\gcd(i,75)=1$, then so is $\gcd(75-i,75)$. Obviously, sum of the numbers in each pair is 75; as for the number of pairs, it is half the number of numbers less than 75 and coprime to it, which is given by the totient function.