If we assume that x is a number of skiers, then there will be x + 1250 snowboarders. We know that the ratio is 1:2, so we can write it down as:

After cross multiply we'll get:
2x = x + 1250 / -x (both sides)
x = 1250
So there are 1250 skiers and 1250 + 1250 = 2500 snowboarders