m = man
w = woman
a = age difference

First we know their ages add up to 91:
m + w = 91

Second, we know there is an age difference of a:
m – a = w

Third, he is twice the age of her when he was her age now:
So when he was her age now, he was (m-a) which that means she was (w-a)
m = 2 * (w – a)

Now we have 3 equations and 3 variables.  Solve how you’d like.  Below is an example.
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Solving for m in terms of a:
m = 2 * ((m – a) – a)
m = 2 * (m – 2a)
m = 2m – 4a
m = 4a

Solving for w in terms of a:
4a – a = w
w = 3a

Solve for a:
4a = 91 – 3a
7a = 91
a = 13

Solve for m and w:
m = 4a
m = 4 * 13
m = 52

w = 3a
w = 3 * 13
w = 39