algorithm - how many times will a function be executed? -

i have piece of code

for ( = 0; < n ; i+=3 ){    foo();    if( % 5 ==0) foo(); } 

and decide how many times foo() executed. have options

ceil(n/3)+floor(n/5) ceil(n/3)+ceil(n/5) floor((n+2)/3)+floor((n+14)/15) floor(n/3)+floor(n/5) floor(n/3)+ceilt(n/5) 

what right way deciding right outcome? outer loop executes foo() outside of condition n/3 times , how decide how many times condition execute function?

just make easier understand can split cycle 2:

for ( = 0; < n ; += 3 ){    foo(); }  ( = 0; < n ; += 15 ){    foo(); } 

for natural n , k cycle (i=0; i< n; += k) run 1 + floor((n - 1)/k) times.

this way total number of runs be: 1 + floor( (n-1) / 3) first , 1 + floor( (n-1) / 15) second cycle is:

2 + floor((n-1) / 3) + floor((n-1) / 15)