The point megamarket sold 450 organic pears last season. this season they only sold 360 what was the percent decrease in sales

First, you would need to determine the difference between last season's pear sales and this season's pear sales.  
450 - 360 = 90

Next, you would need to create an equation to determine what percentage 90 is of 450. Let x represent the unknown sales decrease percentage. 
=

Cross multiply. 
450x = 9,000

Now, all you would need to do is isolate the x. To do this, you would divide both sides of the equal sign by 450. 
x = 20
x = 20%

The percent decrease was 20%.

I hope this helps!