X1 x X1 (any 2 number with units digit 1)

Formulas :

First digits = __ x __

Second digits = __ + __

Third digits = 1

Example :

31 × 51

First digits = 3 × 5

= 15

Second digits = 3 + 5

= 8

Third digits = 1

So, 31 × 51 = 1581