An alloy of gold and silver weighs 50 g. It contains 80\% gold. How much gold should be added to the alloy so that percentage of gold is increased to 90?
(a)30 g
(b)40 g
(c)50 g
(d)60 g
Answer
Answer (as printed): C
Explanation
Quantity of gold in the alloy = 80\% of 50 g $$=\left(\frac{80}{100} \times 50\right) \mathrm{g}=40 \mathrm{g} .$$ Let $x g$ of gold be added. Then, $\frac{40+x}{50+x}=\frac{90}{100}=\frac{9}{10} \Rightarrow 400+10 x=450+9 x \Rightarrow x=50$.