The probability that the 13^th day of a randomly chosen month is a Friday is P then  $840P-1=\_\_\_\_\_\_\_$

 P (Chosen a month)  $=\frac{1}{12}$
There are '7'  possible ways in which the month can start and it will be a Friday on the 13^th day if the 1^st day of month is a Sunday whose probability is  $\frac{1}{7}$
 
$\therefore Req=\frac{1}{12}.\frac{1}{7}=\frac{1}{84}$