\textbf{Unchanged Total}  There are 90 more boys than girls in a school.  The ratio of the number of girls who can swim to the number of girls who can not swim is 31 : 49.  The number of boys who can swim is 120 less than the number of boys who can not swim.  If there are 1690 pupils in the school, how many pupils can not swim?~ 
Overseas Competition->Knowledge Point->Calculation Modules->Ratios and Proportions