Calculate number of terms divisible by a and by b and by c then , remove the terms which is are divisible by both a and b , both b and c , both c and a and then add which are divisible by a and b and c
return ( ( num / a ) + ( num / b ) + ( num / c ) - ( num / lcm ( a , b ) ) - ( num / lcm ( b , c ) ) - ( num / lcm ( a , c ) ) + ( num / lcm ( a , lcm ( b , c ) ) ) ) ; }