Question

22. An alloy of gold and silver weighs 80 gm. It contains
72.5% gold. How much gold should be added to the
alloy so that a new alloy contains 25% silver?
(1) 10g
(2) 8g
(3) 169
(4) 4g
(5) None of these​

Answer 1

Answer:

Initial quantity of Gold = 50 * 80% = 50 * 80/100 = 40 gms. So, initial quantity of Silver = 50 - 40 = 10 gms.

Now, if only Gold is added to make percentage of Gold increased to 90% then, Silver remains at 10 gms only. In final quantity, Silver percentage is 100 - 90 = 10%. Hence, total quantity of alloy metal = 10/ 10% = 10 * 100/10 = 100 gms. So, in the final alloy, quantity of Gold = 100 -10 = 90 gms. Hence, Gold to be added = 90 - 40 = 50 gms Ans.

Answer 2

Answer:

Gold in 50g of alloy

= 80×5010=40g

Let x g gold must be added.

Now, according to the question,

40+x50+x=90100

=>100(40+x)=90(50+x)

=>10(40+x)=9(50+x)

=>400+10x=45+9x

=>80×5010=40g

=>x=450−400=>x=50g