Brass is an alloy formed using copper and zinc in the ratio 3: 2 based on weight. if 120g of copper are needed for a certain piece of brass, how much zinc is needed?

Hey There :)

We can solve by 2 ways ( The 2nd way might be easier )

→ To make 1 brass alloy, the ratio of copper to zinc is 3 : 2
Brass = copper + zinc
          =       3     :    2
                   3x   :    2x
   
Copper needed is 120g
    ↓
3x = 120g
Divide by 3 to isolate x
 
x = 40

Ratio of Zinc is 2

substitute 40 in place of x
2x = 2 ( 40 ) = 80g

Or 

→ 3     :    2
     120 :    p

Cross multiply
3p = 120 x 2
3p = 240

p = 80g

80g of Zinc will be needed with 120g of Copper