Math

1/10=10

10/10-3/10=7/10=70

2/5 x 70 = 28

 

70-28=$\boxed{42}$

First let's find how many people ordered breakfast and how many people ordered drive through.

 

Breakfast = 3/10 * 100 = 30 people

Drive through = 2/5 * 70 (remaining people) = 28

 

 

Now we know that 28 + 30 people ordered either drive through or breakfast, or 58 people did.

 

Now we can subtract 58 from 100:

 

100 - 58 = 42

 

So, 42 people ordered desert.