Approx - 10%
72 students signed up for canoeing + the 23 students who signed up for trekking + the 13 students who signed up for both = 108 students
there are 120 students total, so you have to subtract 120 - 108 = 12
Now to find the percentage, you have to divide the number of students who didn't sign up for either, by the total
which is .1 = 10/100 = 10%