Question 999995
Price is a concentration.  Percent is also often used as a concentration.  Look at another example demonstration using a single variable for one of the coffees.  


<a href="https://www.youtube.com/watch?v=-REjxT9KCzQ">video example same form as your exercise but just a different example.  ONE variable used; two-part mixture.</a>


All two-part mixture problems fitting this form can be handled this way  (although you can also choose to use two variables instead of just one variable.)