Cupro-nickel coins are minted by mixing copper and nichel in the ratio 4: 1
a) how much copper is needed to mix with 20kg of nickel?
b) how much nickel is needed to mix with 20kg of copper?

Copper:   Nickel =  4:1

a) How much copper needed for 20 kg nickel
                 C:N = 4: 1 
                 C:20 = 4:1

C/20 = 4/1       Cross Multiply
C*1 = 4*20
C = 80     

80kg Copper would be needed.

b) How much Nickel would be needed for 20 kg of Copper
                 C:N = 4: 1 
                 20:N = 4:1

20/N = 4/1       Cross Multiply
4*N = 1*20
4N = 20   Divide both sides by N.  

N = 20/4 = 5

5kg Nickel would be needed.