if 12" =1' then 3/8" = ?'

You can set up a proportion:  12" / 1'  =  (3/8)" / x

Cross multiply:  12 · x  =  1 · (3/8)

Therefore:            12x  =  3/8

Divide by 12:            x  =  (3/8) ÷ 12

To divide, multiply by the reciprocal:  

                               x  =  (3/8) · (1/12)

                               x  =  3 / 96  =  1/32' (note that this answer is in feet)