From 1996 to 1999 I had the privilege to teach at an Industrial school for girls, (milder form of the old reformatories). Below is one grade 10 girl's answer as to what National income is. This was part of the November 1997, gd 10 Business economics test paper.
This photo really needs to be enlarged.
Click to enlarge.
I don't think anyone could disagree with her answer.