Prep Questions...please help me

is the answer 1/8?

1/3.....................please explain the answer.............

4 envelops, 4 letters

1 right, 3 wrong.

probability first one goes into correct envelop = 1/4
probability second one goes into incorrect envelop = 1-1/3 = 2/3
probability third one goes into incorrect envelop = 1-1/2 = 1/2
the fourth one will automatically will go into incorrect envelop = 1

probability = 1/4*2/3*1/2 = 1/12


Now we have four cases i.e second going into correct, then third one going into correct envelope and lastly fourth one going into correct one.

Thus,

total probability = 1/12 + 1/12 + 1/12 + 1/12 = 4/12 = 1/3

Hope this helps