Step-by-step explanation:
We see that the ingredients and their respective measurements serve 6 people. There are numerous ways you can solve this, here's one way.
You need the 15 servings and you have the recipe for 6. You can use these values to create a scale factor. To do this, you just divide the intended number of servings by the number of servings the recipe gives you.
This leads to:
15 servings / 6 servings = 2.5
Since this scale factor has no units (the unit of "servings" cancels out through division), you can apply it to all measurements. Meaning, you can multiply each measurement by the scale factor of 2.5.
This gives you:
75 g butter
2.5 onions
5 tablespoons flour
3 liters tomato juice
1200 ml milk
