If the ratio is 1:2 and there are 1250 more snowboarders than skiers, it means that the number of snowboarders is two times 1250 (it's the only way to keep the ratio).
1:2 = 1250:2500
There are 1250 skiers and 2500 snowboarders (because 2500 is two times more and 1250 more at once).
