Answer:
7 cups of soda
Step-by-step explanation:
If we call the number of cups you buy by "x", we have that the price for the refillable cup P1 is:
P1 = 6 + 0.79*x
And the price for the other option is:
P2 = 1.69*x
To find how many sodas do you need to buy to make it work buying the refillable cup, you just need to equate the two prices and find the value of x:
P1 = P2
6 + 0.79*x = 1.69*x
0.9*x = 6
x = 6/0.9 = 6.6667 cups
So, you need to buy at least 7 cups of soda to make the refillable cup worth it.