I believe it is the 28th person that will get both
<span> 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</span>
Take your original equation and replace the variables with their corresponding variables. As a result, you get the equation
F=9(-6)+2(7)
F=-54+14
F= 40
