Question

Suppose that 5 invitations to a graduation party are written and 5 envelopes are addressed. Accidentally the invitations are randomly inserted into the envelopes and mailed without checking the addresses. What is the probability that all the invitations will be inserted into the correct envelopes ?

Answer 1

the 1st letter can be put into any of the 5 envelopes,
the 2nd letter in any of the 4 r3maing envelopes, & so on,
giving a total of 5*4*3*2*1 = 120 ways,
while there is just 1 way to insert all letters into correct envelopes

Pr = 1/120