If 8 letters are placed in 8 addressed envelopes, then the probability that exactly 3 letters go into correct envelopes and none of the remaining letters go into correct envelop is

n(s) = Total arrangements of 8 letters in 8 addressed
envelops =$\overline{)8}$
No. of ways in which exactly 3 letters can be kept in correct envelops $=\left({ }^8{\mathrm{C}}_3\times {\mathrm{D}}_5\right)=56\times 44$

$\left(\text{ here }{\mathrm{D}}_5\right.=5!\left(\frac{1}{2!}-\frac{1}{3!}+\frac{1}{4!}-\frac{1}{5!}\right)=60-20+5-1=44$)is number of disarrangements of 5 letters), 

Required probability $=\frac{56\times 44}{8!}=\frac{11}{180}$