Twenty-one out of 70 customers at the restaurant ordered dessert. what percentage of the customers did not order dessert?

First, you would need to determine how many customers ordered dessert. 
70 - 21 = 49

Next, you would need to create an equation for this problem. Let x represent the unknown percentage. 
=

Cross multiply. 
70x = 4,900

Now, all you would need to do is isolate the x. To do this, you would divide both sides by 70. 
x = 70
x = 70%

So, 70% of the customers ordered dessert. 

I hope this helps!