1.  An alloy of gold and silver weights 50g. It contains 80% gold. How much gold should be added to the alloy, so that percentage of gold is increased to 90 ?
A. 50g
B. 60g
C. 30g
D. 40g

Answer: Option A

Explanation:

Gold in 50g of alloy = (80 x 50)/100 = 40 g

Let W gram gold must be added.

Now, according to the question,

 (40 + W) / (50 + W) = 90/100

 100( 40 + W) = 90 (50 + W)

 10(40 + W) = 9 (50 + W)

 400 + 10W = 450 + 9W

 W = 450 - 400

W = 50g

Thus, 50 g of gold must be added to make it 90%