Let success is to select a smart phone and it is for office purpose. ∴ p=
1
2
,q=1−p=1−
1
2
=
1
2
Now, probability that out of 10 users exactly 2 of them use for office purpose = Probability of setting exactly 2 success out of 10 trial =10C2.p2q8[∵[P(X=r)=nCrpr.qn−r] =10C2.(