Answer
$35$
Work Step by Step
The number of unordered sets that contain three objects chosen from seven equals to the number of combinations of 7 items taken 3 at a time : $C(7,3)=\frac{7!}{3!(7-3)!}=\frac{7!}{3!*4!}=\frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1}=7*5=35$
