Alright! We know that the total number of students is 100%. So, if 65% of the students brought a hot lunch, in order to find how many students brought a packed lunch you subtract it from 100%:

[100 – 65] = 35

So 35% of the students brought a packed lunch. Now, in order to find the fraction of students you simply put 35% of students over the total percent of students (100%)”

[35/100]

Then you simply:

[35/100] ⇒ [7/20]

The answer is 7/20ths of the students brought a packed lunch.

More: 
Decimal form – .35
Percentage form – 35%