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?

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%