A gold chain with an original price of $150 is on sale for $87 what is the percent of the discount

The answer is 30%.

This is because 60-42 is 18.  
Now our new problem is what percent of 60 is 18?

 x=18%. 
Now we have two simple equations:
1) 60=100%
2) x=18%
Where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
60/x=100%/18%
x=18%

Hope this helps!!