this is the theorem @`JEET
if a cyclic group Gis generated by an element a of order n,then $a^{m}$ is a generator of G if and only if the greatest common divisor of m and n is 1 that is m & n are relatively prime
cyclic group of order 10
now in this 1,3,7,9 are the nos which are relatively prime to 10
hence 4 generators!!