Percentage Practice Test 6 with solutions

Weight of an alloy of gold and silver = 50 gms
Per cent of gold = 80%
= $50\times \frac{80}{100}=40$ gms
If some Gold is added to the alloy, then the percentage of gold is increased to 90%.
$\frac{40+\mathit{x}}{50+\mathit{x}}\times 100=90$
$\left(40+\mathit{x}\right)100=4,500+90\mathit{x}$
$4,000+100\mathit{x}=4,500+90\mathit{x}$
$100\mathit{x}-90\mathit{x}=4,500-4,000$
$10\mathit{x}=500$
$\mathit{x}=50$
So, 50 gms gold should be added to the alloy.