Look, any number which is in the form of (a*120) i.e. multiple of 120 will be divisible by 120 right ?
Here, 1!=1
3!= 6
5!= 120 (5*4*3*2*1)
7!= (7*6)*120 i.e. multiple of 120
Now, take an example for understanding purpose (30+1)/2 => reminder will be 1. since 30 is divisible by 2 but 1 is not .
similarly, here after (1!+3! ) all are divisible by 120.
hence ans. is 1+6= 7