An alloy of gold and silver weighs 50 gms. 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
$$=80\times\frac{50}{100}=40g$$
Let x g gold must be added.
Now, according to the question,
$$40+x=\frac{90}{100}\left(50+x\right)$$
$$x=50g$$
Thus, 50 g of gold must be added to it to get 90%.
Thus, the correct option is A.
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