Dan must be from outer space. They have teo-digit numbers over there.

If Dan found only a two-digit number, then he may be from our earth.

Two-digit number: xy ---not x*y, but (10x +y).

xy < 50 ----(i)

x+y = 12 ---(ii)

x -y = 4 -----(iii), if x is greater than y.

Add (ii) and (iii),

2x = 16

x = 16/2 = 8

Substitute that into (ii),

x +y = 12

y = 12 -8 = 4

So xy = 84, which is not less than 50, hence, wrong.

So we try the next option for (iii).

y -x = 4 ----(iiia), y is greater than x.

y = 4 +x

Substitute that into (ii),

x+y = 12 ----(ii)

x +(4+x) = 12

x +4 +x = 12

x +x = 12 -4

2x = 8

x = 8/2 = 4

Substitute that into (ii),

4 +y = 12

y = 12 -4 = 8

Then, xy = 48, which is less than 50.

Therefore, Dan found the number 48. -----answer.