Brown rocks cost $5 per pound. Beige rocks cost $11 per pound. How many brown rocks are needed to create a 10 lb mixture that costs $6.80 per pound?

Question

anonymous · Answer

I think this question is much like the last one which involved the distances and their speed.

anonymous · Answer

okay so how would I set it up? x+y=z?

anonymous · Answer

I always first try to use some dimensional analysis.  Have you heard of that?

anonymous · Answer

no what is it?

anonymous · Answer

In this problem our dimensions are $$\frac{ dollars }{ pound }$$
But we need to know how many rocks.  Is there any information that says 1 rock = x pounds?  
Also, knowing the dimensions allows us to pick how we want to solve for things.  So if we wanted pounds, well we'd know that we would have to flip the reciprocal to get pounds up top.  Does that make sense?

anonymous · Answer

yeah, so would these be my equations,x+y=10 and 5x+11y=68 and I would use substitution?

anonymous · Answer

At this point you've just created what's call a linear system of equations.  You have two equations and two unknowns.  You can use one to help solve the other.

anonymous · Answer

You can use any method that makes the most sense to you.  Substitution is probably going to be easier here though.

anonymous · Answer

okay thanks i got it from here

anonymous · Answer

Rock on man!