Let’s say that the lemonade power is (p) and the water is (w).
Let’s also say that one cup of lemonade is (L)
Since you need 3 cups of (p) and 7 cups of (w) to make 1 of (L), the equation would be f(L) = 3p + 7w
The (L) would be input of how many cups you need
So if you need 5 cups, use 5 as your input
F(5) =3(5)+7(5)
3x5=15
7x5=35
You need 15 cups of Lemonade powder
And 35 cups of water