A jeweler has five rings, each ring weighing 36 g, made of an alloy of 10% silver and 90% gold. She decides to melt down the rings and add enough silver to reduce the gold content to 75%. How much silver should she add?
If she add x grams, then since 5 rings weigh 180 g, she needs
0.10*180 + x = 0.75(180+x)