Answer:
The merchant should mix 53.33 pounds of cheaper cacao and 26.67 pounds of costlier cacao.
Step-by-step explanation:
A merchant wants to blend 80 pounds of cacao worth $9.00 a pound from two kinds: one at $11.40 a pound and the other at $7.80 a pound.
Let the number of pounds of cheaper cacao be x
Let the number of pounds of costlier cacao be y
The merchant wants total pounds = 80
So, first equation becomes:
or
Now values of per pounds become
and 
As the merchant wants the mixture worth $9 so second equation becomes:

=> 
Substituting
in second equation, we get

=> 
=> 
pounds
We know, 
=>
pounds
So, The merchant should mix 53.33 pounds of cheaper cacao and 26.67 pounds of costlier cacao.
We can check this:
dollars