I reckon that each women has an equal chance of picking her own husband ie 1/40 or 39/40 chance of not picking her own husband.
I think the probability of at least one woman picking her own husband is
1 minus the probability of no women picking their own husbands.
ie 1 - (39/40)^40
=
1 - 0.363
= 0.637 to three decimal places
My son says otherwise and came up with some complex arrangement similar to your book but couldn't be bothered to work out the answer. So what does the book calculation actually come to? ?anywhere near 0.637?
