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