for 1 serving the ratio is 1 : 2
for 2 servings the ratio is 2 : 4
so by looking at the two ratios above you can see that
for two servings the ratio 1 : 2 is multiplied by two
so the answer for 5 servings is 5 : 10
so no matter the number of servings when you simplify the ratio the ratio you should obtain (as in this case) should be 1:2 which the ratio for 1 serving.
hope this helped :)