The ven diagram of the problem :-
It is given ,
$|S\cup I|=35$
$|S\cap_{}^{} I|=20$
So number of Student who are doing exactly one activity is the red and violet portion , which we can easily found by ,
$|S\cup I|$ – $|S\cap_{}^{} I|$ $=35-20=15$
So, $15$ student agreed to do exactly one activity.
No of Student decline to participate is = total number of student – no of student agreed to participate at least one of the activity
$=60 – $$|S\cup I|$
$=25$
So ,$25$ student decline to do any activity.