Question

How much orange juice at 45 a liter should be mixed with grapefruit juice worth 35 a liter to make 200 liters of a mixture worth 40 a liter?

Answer 1

The amount of liters is 200,if the orange juice is x than grape juice. 200-x
45x+35(200-x)=200*40
45x+7000-35x=8000
10x=1000
x=100

Answer 2

X - liters of $45 per liter
y - liters of $35 per liter
We need 200 liters worth of $40 per liter

A) x + y = 200 liters
B) .45x + .35y = .40 * 200
B) .45x + .35y = 80 We multiply equation A by -.45
A) -.45x  -.45y = -90 liters Then we add A and B
-.10y = -10
y = 100
and therefore x = 100
So we need 100 liters of $45/liter
and 100 liters of $35/ liter

Source:
http://www.1728.org/mixture.htm